Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments: Volume 2: Corrosion Protection Measures

By Andreas Momber

Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments: Volume 2: Corrosion Protection Measures offers the first comprehensive review on corrosion and corrosion protection of offshore wind power structures. The book extensively discusses corrosion phenomena and corrosion types in different marine corrosion zones, including the modeling of corrosion processes and interactions between corrosion and structural stability. The book addresses important design issues, namely materials selection relevant to their performance in marine environments, corrosion allowance, and constructive design. Active and passive corrosion protection measures are emphasized, with special sections on cathodic corrosion protection and the use of protective coatings.

Seawater related issues associated with cathodic protection, such as calcareous deposit formation, hydrogen formation, and fouling, are discussed. With respect to protective coatings, the book considers, for the first time, complete loading scenarios, including corrosive loads, mechanical loads, and special loads, and covers a wide range of coating materials. Problems associated with fouling and bacterial-induced corrosion are extensively reviewed. The book closes with a chapter on recent developments in maintenance strategies, inspection techniques, and repair technologies. The book will be of special interest to materials scientists, materials developers, corrosion engineers, maintenance engineers, civil engineers, steel work designers, mechanical engineers, marine engineers, chemists, and coating specialists.

Offshore wind power is an emerging renewable technology and a key factor for a cleaner environment. Offshore wind power structures are situated in a demanding and challenging marine environment. The structures are loaded in a complex way, including mechanical loads and corrosive loads. Corrosion is one of the major limiting factors to the reliability and performance of the technology. Maintenance and repair of corrosion protection systems are particularly laborious and costly.

• Explores the literature between 1950 and 2020 and contains over 2000 references
• Offers the most complete monograph on the issue
• Covers all aspects of corrosion protection in detail, including coatings, cathodic protection, corrosion allowance, constructive design, as well as maintenance and repair
• Delivers the most complete review on corrosion of metals in marine/offshore environments
• Focuses on all aspects of offshore wind power structures, namely foundations, towers, internal sections, connection flanges, and transformation platforms

• Language: English
• Publisher: Academic Press
• Release date: Apr 7, 2024
• ISBN: 9780323857451

Andreas Momber

Andreas Momber is the head of R&D of a surface protection service company which focusses on marine applications. He has over twenty years experience in surface protection and has managed numerous funded projects with respect to marine and offshore wind corrosion. Dr. Momber is a regular contributor to relevant conferences, workshops, meeting etc. on offshore wind power. Research on universities in the US, Australia, Germany and UK on surface technology. He has published over 200 scientific papers, as well as several well-known works including: Hydroblasting and Coatings of Steel Structures and Hydrodemolition of Concrete Surfaces and Reinforced Concrete. Dr. Momber is an active member of NACE, SSPC and GFKORR.

Book preview

Front Cover for Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments - Volume 2: Corrosion Protection Measures - 1st edition - by Andreas Momber

Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments

Volume 2: Corrosion Protection Measures

Andreas Momber

Muehlhan AG/Muehlhan Holding GmbH, Hamburg, Germany

Privatdozent Faculty of Geo-Resources and Materials Technology, University of Aachen, Aachen, Germany

Table of Contents

Cover image

Title page

Copyright

Foreword

Chapter 4. Material selection

Abstract

4.1 General recommendations

4.2 Low-carbon (unalloyed) steel

4.3 Stainless steel

4.4 Aluminum alloys

4.5 Zinc

4.6 Copper and copper alloys

4.7 Nickel and nickel alloys

4.8 Welds and weld overlays

4.9 Fiber-reinforced composites

4.10 Bolts and screws

4.11 Concrete

Chapter 5. Constructive design

Abstract

5.1 General

5.2 Sealing and climatization

5.3 Accessibility

5.4 Avoidance of gaps, joints, and overlapping connections

5.5 Precautions against deposits and the accumulation of electrolyte

5.6 Steel preparation

5.7 Bolted connections

5.8 Bimetal coupling

5.9 Handling, transport, and assembling

5.10 Vapor corrosion inhibitors

Chapter 6. Corrosion allowances

Abstract

6.1 Basics

6.2 Calculation of corrosion allowance

Chapter 7. Cathodic corrosion protection

Abstract

7.1 Basics

7.2 Cathodic protection with galvanic anodes

7.3 Cathodic protection with impressed current

7.4 Formation of calcareous deposits

7.5 Simulation of cathodic protection systems

7.6 Cathodic protection for internal sections of offshore wind power structures

7.7 Splash zone and tidal zone protection

7.8 Cathodic protection of chains

7.9 Cathodic protection in the seabed

7.10 Cathodic protection of special design features

7.11 Cathodic protection with thermally sprayed aluminum

7.12 Cathodic protection with AlZn alloys

7.13 Alternative methods

7.14 Coating breakdown factor

7.15 Cathodic disbonding of coating

7.16 Hydrogen-induced stress cracking during cathodic protection

7.17 Effects of cathodic protection on stress corrosion cracking

7.18 Effects of cathodic protection on corrosion fatigue

7.19 Alternating current corrosion under cathodic protection

7.20 Effects of cathodic protection on fouling

7.21 Prevention effect parameters

Chapter 8. Protective coatings

Abstract

8.1 Basics

8.2 Organic coating systems

8.3 Metal coatings

8.4 Other metal coatings

8.5 Wrapping, insulations, linings

8.6 Coatings for bolts and screws

8.7 Coating systems for mechanical loads

8.8 Coating systems for special loads

8.9 Antifouling coatings

8.10 Recommended coating systems for offshore wind power structures

8.11 Coating specifications for offshore wind power structures

8.12 Testing standards for marine and offshore coatings

8.13 Coating failures

8.14 Coating lifetime models

8.15 Coating selection

Chapter 9. Inspection, maintenance, and repair

Abstract

9.1 Maintenance strategies

9.2 Condition assessment

9.3 Inspection

9.4 Quantification of coating deterioration and steel corrosion

9.5 Sensors

9.6 Repair coatings

Bibliography

Further reading

Index

Copyright

Academic Press is an imprint of Elsevier

125 London Wall, London EC2Y 5AS, United Kingdom

525 B Street, Suite 1650, San Diego, CA 92101, United States

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

Copyright © 2024 Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Publisher’s note: Elsevier takes a neutral position with respect to territorial disputes or jurisdictional claims in its published content, including in maps and institutional affiliations.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-323-85744-4

For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Megan Ball

Acquisitions Editor: Edward Payne

Editorial Project Manager: Joshua Mearns

Production Project Manager: Surya Narayanan Jayachandran

Cover Designer: Matthew Limbert

Typeset by MPS Limited, Chennai, India

Foreword

The energy sector is undergoing an important transition to meet the world’s energy needs, reduce greenhouse gas emissions, support climate change mitigation, and protect our planet for future generations. A major element of the energy transition is the integration of large-scale renewable energy sources, including wind energy, in the energy mix of countries.

For millennia, humanity has harnessed the wind for mobility with sailboats, whereas the wind has been used for centuries to perform mechanical work using large surfaces that interact with the passing airflow. More recently, wind power has achieved technological maturity, and, along with solar energy and bioenergy, wind energy is poised to play a significant role in the energy transition.

In support of the global research community and the commercial wind energy sector, the Wind Energy Engineering Series publishes research and application-oriented books on the overarching subjects related to wind energy engineering, focusing on scientific and technical content that supports all stages of research and application.

While still in its infancy, offshore wind energy is the next focus in developing wind power as a reliable and affordable source of renewable energy. Building on the experiences of onshore wind power developments, offshore wind energy is constrained by demanding and challenging work environments, with corrosion being one of the major limiting factors to the reliability and performance of offshore wind power structures and equipment.

Building on his broad expertise in corrosion research and industrial applications, Dr. Andreas Momber has produced a comprehensive two-volume set on corrosion and corrosion protection, specifically targeted for wind power structures in marine environments. Well anchored in the theoretical and fundamental concepts of corrosion, the two-volume set transposes these concepts to design and operational issues specifically applied to offshore wind power systems.

In Volume 2, Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments: Corrosion Protection Measures, Dr. Momber addresses the important topic of the selection of materials for power structures installed and operating in corrosive environments, along with the design issues to consider in wind power systems, including the corrosive loads. Various cathodic corrosion protection technologies and protective coatings are covered under wind power applications. This book concludes the two-volume set with the specific, and so important, operational issues of inspection techniques, maintenance strategies, and repair technologies related to corrosion. For its part, Volume 1 focuses on the fundamental aspects of corrosion, corrosive loads, and corrosion models in wind energy applications.

Unique in the scientific and technical literature on wind energy, the two-volume set of Corrosion and Corrosion Protection of Wind Power Structures in Marine Environments constitutes a masterpiece on corrosion of wind power structures operating in marine environments. There are no doubts that this contribution by Dr. Andreas Momber, a leading authority on the subject of corrosion, will be a reference for the offshore wind energy research, manufacturing, development, and operational communities.

Prof. Yves Gagnon PEng, DSc

Université de Moncton, Canada

Series Editor, Wind Energy Engineering Series

Chapter 4

Material selection

Abstract

This chapter provides a review on the principles and methods for the selection and evaluation of materials used in marine and offshore environments. Particular attention is spent on low-carbon steel, stainless steel, aluminum alloys, copper, and zinc. The selection of materials for bolts as well as the performance of fiber-reinforced plastics is also considered.

Keywords

Material groups; welds; bolts; PREN; alloys; dezincification and dealuminization

4.1 General recommendations

4.1.1 Selection criteria

Materials for offshore wind power constructions, particularly steels, are predominantly selected according to their strength properties, such as tensile strength, yield strength, elongation, Young’s modulus, fatigue life, and impact strength. Material selection in relationship with corrosion, however, is the use of corrosion-resistant materials. Examples for offshore wind power constructions may include the use of carbon steel, stainless steel for bolting and fastening devices, aluminum (alloys), copper (alloys), and the use of fiber-reinforced plastics for gratings (DNVGL, 2016a). ISO 12944-1 (2018) recommends the use of corrosion resistant materials for structural components which are exposed to corrosion stresses and which are not longer accessible for corrosion protection measures after assembly.

The general philosophy of a material selection process for steel structures shall reflect a number of issues, namely the following (Norsok, 2014):

• design lifetime.

• cost profile.

• inspection and maintenance philosophy.

• safety and environmental profile.

• failure risk evaluation.

For offshore structures, as a minimum, the following criteria shall be considered (Norsok, 2014):

• corrosivity.

• design life and system availability requirements.

• failure probabilities, failure modes, and failure consequences.

• inspection and corrosion monitoring.

• access for maintenance and repair.

Kappenthuler and Seeger (2021) presented a holistic approach for the selection of suitable metals for marine constructions. They defined four assessment categories:

• durability.

• sustainability.

• economics and costs.

• future availability.

The durability category covered the following attributes:

• corrosion resistance (structural damage in the splash zone).

• resistance to degradation by marine organisms.

• fatigue resistance.

• resistance to stress corrosion cracking.

• ultraviolet resistance.

• moisture resistance.

To compare the performance of different metals, a functional unit was defined: the weight required to produce a 1.0-m long beam with a square cross section that is able to resist a tensile load of 5000 kN. Results of the selection procedure are listed in Table 4.1. Although carbon steel received the lowest durability score, this was compensated by a low price and a high availability. The final ranking of the investigated metals was as follows: carbon steel>titanium alloy>aluminum alloy>NiCu-alloy>stainless steel.

Table 4.1

aFunctional unit.

Fig. 4.1 illustrates the effects of material selection on the corrosion rate of two bolt materials exposed to a chloride-containing environment. The material not suitable to this type of environment corroded, whereas the high-alloy steel remained in good condition. For the outside equipment of offshore wind power structures, DNVGL (2015d) recommends using copper (or other corrosion-resistant material). However, for costs reasons, offshore wind power structures are mainly made from corrosion-sensitive construction steels (compare, among others, Martin and Schröder, 2005).

Figure 4.1 Effects of material selection in a chloride-containing environment (Fischer and Burkert, 2007).

As a general rule, (DNVGL, 2016a) stated for wind turbines: All corrosion resistant materials to be used shall be specified by reference to a [national or international] material standard defining requirements to chemical composition, mechanical properties and quality control of manufacturing.

With respect to uniform corrosion, Schütze et al. (2016) suggested three corrosion resistance levels. They are defined in Table 4.2.

Table 4.2

4.1.2 Corrosion and alloying

ISO 9224 (2012) suggests the following relationship for the estimation of corrosion:

Equation (4.1)

Here, C is the corrosion attack (either weight loss per unit area or corrosion depth), rcorr1 is the corrosion rate in the first year, tE is the exposure time, and b is a material-environment-specific exponent. With rcorr1=A and b=B, Eq. (4.1) corresponds to the standard power-law model for atmospheric corrosion (see 2.5.2). In terms of material effects, the exponent b is a function of metal alloying elements as follows (ISO 9224, 2012):

Equation (4.2)

Here, bi is a multiplier for the ith alloying element, and wi is the mass fraction of the ith alloying element. Eq. (4.2) basically describes the effect of alloying elements on the protective performance of rust layers formed on the metals. Values for bi for some alloying elements are listed in Table 4.3. However, Eq. (4.2) is valid for nonmarine exposure. In a marine environment, b is assumed to increase due to chloride deposition (see 2.5.2.3). The increase can be approximated as follows (ISO 9224, 2012):

Equation (4.3)

Here, Δb is the increase in b in a marine atmosphere, and Sd is the chloride ion deposition rate in mg/(m² day).

Table 4.3

The effects of alloying elements on the corrosion performance of steels in marine environments depend also on the particular type of environment. Table 4.4 provides examples for seawater immersion and tidal zone exposure.

Table 4.4

4.2 Low-carbon (unalloyed) steel

4.2.1 Selection

4.2.1.1 Corrosion management program

Steels suitable for different offshore wind power structures are listed in Table 4.5. These materials mainly include fine-grained, low-carbon steels. Typical compositions and properties of respective steels are provided in Table 4.6 and in Table 4.7. Low-carbon steels are not resistant against corrosion in corrosive environments, and they must, therefore, be protected against corrosion.

Table 4.5

Table 4.6

aThickness: >16 mm.

bThickness: ≥3 mm.

cThickness: >3 mm.

dTesting temperature: −20°C.

Source: Salzgitter Flachstahl GmbH, Salzgitter.

Table 4.7

aThe sum of Nb+V+Ti shall not exceed 0.22 wt.%.

bMaximum carbon equivalent: CE=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15.

Source: Salzgitter Flachstahl GmbH, Salzgitter.

Norsok (2014) recommended a corrosion management program for carbon steels used in corrosive services, which should as a minimum consist of the following parts:

• definitions of roles, responsibilities, and reporting routines within the organization (manufacturer).

• corrosion risk evaluation.

• planning and execution (methods, location, and frequency) for corrosion monitoring, process parameter monitoring, and water analyses.

• planning and execution of addition of corrosion control chemicals.

• development of procedures for evaluation of corrosion monitoring data and for verification that the corrosion rates and conditions are within acceptable levels (predefined targets).

• definition of consequences and actions if targets are not met.

4.2.1.2 Material groups

With respect to the corrosion of steels in seawater environment, DIN 81249-2 (2013) distinguishes two material subgroups, namely, FE 4 (steel) and FE 5 (iron). The respective corrosion rates are listed in Table 4.8.

Table 4.8

aDepends, among others, on the surface finish.

bLong-term exposure in the North Sea (lower value: after 2 years; higher value: after 5 years).

cIn sea atmosphere (North Sea).

For uniform corrosion, low contents of alloying elements Cr, Cu, Al, and Si are considered to reduce the corrosion rate of low-alloy steels in the immersed zone and in the splash zone. Depending on type and amount of the alloying elements, the corrosion rates of these materials are lowered by factors between 0.2 and 0.5 (DIN 81249-2, 2013).

4.2.1.3 Atmospheric corrosion index

A parameter to characterize the resistance of low-carbon steel against atmospheric corrosion is the Atmospheric Corrosion Index (ASTM G101, 2004):

Equation

(4.4)

In the equation, IA is the atmospheric corrosion index, and all elements are given in %. The higher the index, the greater is the corrosion resistance. The index is valid for maximum values of Cu (0.51%), Ni (1.1%), Cr (1.3%), Si (0.64%), and P (0.12%).

For high-strength low-alloy steels to be used in an atmospheric marine environment, ASTM A690M (2013) defines the following alloy requirements (in wt.%): Ca (0.22), Mg (0.60–0.90), P (0.08–0.15), S (0.04), Si (0.40), Cu (0.50), and Ni (0.40–0.75).

4.2.1.4 Strength requirements for high-strength low-alloy steel

For high-strength low-alloy steel to be used in an atmospheric marine environment, ASTM A690M (2013) defines the following strength requirements: minimum tensile strength: 485 MPa; minimum yield point: 245 MPa; minimum elongation (50 mm): 21%.

4.2.1.5 Linear assessment equations

Hou et al. (2000) and Wang et al. (2008b) investigated effects of alloying on the corrosion of low-alloy steels in different marine environments. In detail, they derived the following equations (with a correctness of 83%). For marine atmospheric corrosion:

Equation

(4.5)

For marine splash zone corrosion:

Equation

(4.6)

For immersion in seawater:

Equation

(4.7)

In the equations, rcorr is the corrosion rate in mm/year, and the elements are given in wt.%. The average corrosion rates were highest in the splash zone, followed by the immersed zone and the atmospheric zone. In detail, however, the corrosion rates depended on the steel alloying. In the immersed seawater zone, the corrosion rate of 17NiCuP was somewhat higher compared with general carbon steel. The corrosion rate of 10Cr2AlMo was slower compared with 17NiCuP. The corrosion resistance was increased by Al and Cr. In the splash zone, the corrosion rate of 17NiCuP was notably lower compared with general carbon steel. The corrosion rate of 10Cr2AlMo was notably higher than 17NiCuP. The corrosion resistance was increased by P, Mo, Cu. In the atmospheric zone, the corrosion resistance was increased by Cu, P, Mo. Fig. 4.2 provides results of the regression equations in comparison with measured corrosion rates.

Figure 4.2 Experimental and predicted corrosion rates for different steels (based on data in Wang et al., 2008b).

Schultze and van der Wekken (1976) analyzed numerous corrosion rate results obtained in marine environments and derived a multiple linear regression function:

Equation (4.8)

Here, Equation is the corrosion rate ratio, rcorrA is the corrosion rate of the alloyed steel, rcorr is the corrosion rate of the steel not containing the alloying element, and Ci are the concentrations of alloying elements in %. Values for ri and Ci for numerous alloying elements are published in Schultze and van der Wekken (1976). Fig. 4.3 shows results for metals immersed in seawater, and Fig. 4.4 shows results for metals for tidal zone exposure. It appeared from the analyses that from the point of view of improving the corrosion resistance of low-alloy steel in seawater, three alloying elements were of special interest. Cr and Mo had an important effect during immersion; Al was effective in the tidal zone and possibly also during immersion. The results in Fig. 4.3 indicated, however, that the influence of Cr additions changed from beneficial to detrimental after about 4 years immersion. The effect of Mo changed after about the same period in the opposite sense. The authors claimed that heavy fouling could have changed the conditions in the rust layer in such a manner that the effect of the alloying elements was changed. The time dependence of the influence of Cr as shown in Fig. 4.4 indicated that conclusions regarding the beneficial effect of this element based on short-term tests could not be relied upon for predictions of long-term influences.

Figure 4.3 Values of ri-coefficients of various alloying elements for full immersion. Schultze, W.A., van der Wekken, C.J., 1976. Influence of alloying elements on the marine corrosion of low alloy steels determined by statistical analysis of published literature data. Brit. Corr. J. 11 (1), 18–24. Reprinted by permission of Taylor & Francis Group LLC.

Figure 4.4 Values of ri-coefficients of various alloying elements for tidal zone exposure. Schultze, W.A., van der Wekken, C.J., 1976. Influence of alloying elements on the marine corrosion of low alloy steels determined by statistical analysis of published literature data. Brit. Corr. J. 11 (1), 18–24. Reprinted by permission of Taylor & Francis Group LLC.

Based on statistical analysis of alloying elements, Blekkenhorst et al. (1986) derived the following multiple linear regression model for the corrosion rate of high-strength steels immersed for 1.6–7.2 years in natural seawater (North Sea, water depth: 45–90 m):

Equation

(4.9)

Here, rcorr is the corrosion rate in mm/year, the first term is the general mean of the corrosion rate in mm/year, and ε is the error. The equation is valid for steels immersed in 45-m deep seawater (North Sea) for 4 years. Chromium and aluminum additions were found to have consistently beneficial effects on the general corrosion rate, even though their interaction was slightly detrimental. The addition of 0.5% Mo was found to be slightly beneficial, while 1.5% Mo was detrimental. In general, carbon showed no significant effect. The improved performance of the Al- and Cr-containing steels, combined with 0.5% Mo, was attributed to the formation of a continuous magnetite layer on the metal surface, showing less porosity and improved adherence owing to the incorporation of alloying elements (Blekkenhorst et al., 1986).

Wang et al. (2008a) derived a model for the pitting corrosion of low-alloy steel in natural seawater (Qingdao, North China coast) by means of a three-parameter Weibull model (see 2.5.25.3). The relationships between corrosion model parameters and steel composition were estimated with multiple linear regression analysis:

Equation (4.10)

Here, Y is the corrosion model parameter, Xi (i=1…k) is the composition content of the steel, and Bi are regression coefficients. Results of the calculations are provided in Table 4.9.

Table 4.9

4.2.1.6 Comprehensive index value

Chen et al. (2020) introduced an index number to evaluate effects of alloying elements on the corrosion potential of 14 low-carbon steels immersed for 5 months in natural seawater (Sanya, South China Sea). The index was estimated as follows:

Equation (4.11)

Equation (4.12)

Equation (4.13)

In the equations, CIV is the comprehensive index value, A is a term for environmental factors, and W is a function of undetermined weight factors. Of the six seawater parameters initially considered, pH-value and salinity were omitted from the model because they did not change considerably during the exposure period (131 days). Finally, four seawater variables (α1 to α4) were included: temperature in °C, specific conductivity in µS/cm, dissolved oxygen in mL/L, and oxidation-reduction potential in mVAg/AgCl. The final assessment model, called CIV-RVR (relevance vector regression) model, was defined as follows (Chen et al., 2020):

Equation (4.14)

In the equation, y is the seawater corrosion potential in mVAg/AgCl, tE is the exposure time in days, CIVavg is the average comprehensive index value in the kth sample, and e1–e15 represent the content of 15 alloying elements: C, Si, Mn, P, S, Ni, Cr, Mo, Cu, Al, Ti, Nb, V, B, and N. The model was found to deliver more accurate results than other artificial intelligence models (Table 4.10). A data mining process enabled a detailed evaluation of the effects of the alloying elements on the corrosion potential. Based on Spearman’s correlation coefficients, Chen et al. (2020) determined seven key alloying elements, namely, Cr, Ni, Mo, P, Cu, Si, and V, and established corrosion laws for low-alloy steels in seawater. Examples for the effects of P and Cu are illustrated in Fig. 4.5. When the Cu-content was below 0.16 wt.%, the corrosion potential of the steel was almost independent of the amount of P. For Cu-contents in excess of 0.16 wt.%, however, the corrosion potential became less negative with increasing P-content at the shorter exposure periods. For all cases, the corrosion potential showed less negative values at longer exposure times.

Table 4.10

Note: ANN, Artificial neural network; COD, coefficient of determination; SVR, support vector regression.

Figure 4.5 Effects of copper, phosphorus, and immersion time on the seawater corrosion potential on a low-alloy steel (Chen, L., Fu, D., Chen, M., 2020a. Modeling and mining dual-rate sampled data in corrosion potential online detection of low alloy steels in marine environment. J. Mater. Sci. 55, 13398–13413. Reprinted by permission of Springer Nature).

4.2.1.7 Interaction between alloy elements and carbon

AI-Hajji and Reda (1992) investigated effects of alloying elements on the corrosion of carbon steels (carbon content between 0.1% and 0.4%) in natural seawater (Arabian Gulf; pH=8.4; salinity: 40.7%). They found the effects of Cr, Co, and Mo to interact with the carbon content of the steels. Higher chromium contents (4.0%) reduced the corrosion rates. Cobalt (between 1.0% and 3.0%) played a neutral role, whereas molybdenum (>1.0%) played a major role in reducing the corrosion rate, particularly in steels with higher carbon contents.

4.2.1.8 Interaction between alloy elements and corrosive environment

Takamura et al. (1971) performed an extensive study into the effects of alloying elements on the corrosion resistance of low-alloy steel in different simulated seawater zones. Samples were exposed for 12 months to a natural exposure site (Amagasaki, Japan) as well as to intermitted corrosion tests (8 h dry-wet cycles in artificial seawater; 16 h exposure in humid air). Results from the latter tests are provided in Fig. 4.6. The effects of specimen size, thus of exposure situation, on the performance of the differently alloyed steel, can be recognized. Results from the site exposure tests are listed in Table 4.11. Effects of alloying elements on the corrosion resistance were dependent on the conditions of drying and wetting. In environments with relatively longer dry periods (marine atmosphere, splash zone), the addition of Mn, Cu, P, and Cr manganese improved the corrosion resistance of the steel. In environments with longer wet periods (lower splash zone, tidal zone), the addition of P improved the corrosion resistance considerably, whereas Cr showed adverse effects.

Figure 4.6 Corrosion of long-size and short-size specimens of four differently alloyed steels after 2 months intermittent immersion. Source: Takamura, A., Akawa, K., Fujiwa, K., Hirose, H., 1971. Effects of alloying elements on the corrosion resistance of low alloy steel in the sea-water splash zone. Trans. lSIJ 11, 299–306.

Table 4.11

Based on tests in a simulated tidal tank with natural seawater (Qingdao, China), Hou (1986), too, noted effects of corrosion zones on the performance of alloying elements. In the splash zone, Ni, Cu, P, and Mo increased the corrosion resistance of steel, whereas Cr, Al, Mo, P, and W were effective to improve the corrosion resistance in the atmospheric zone.

4.2.1.9 Sn-bearing steel

Kamimura et al. (2012) investigated the atmospheric corrosion of Sn-bearing steel, and they found that Sn-bearing steel exhibited a superior atmospheric corrosion resistance in a cyclic corrosion test (6 hours humid stage: 50°C and 100% RH; 15 minutes immersion: 0.3 wt.% NaCl, pH=8.0; 17.75 hours drying: 60°C and 50% RH). The order of corrosion resistance (from best to worst) was Fe0.6Sn>Fe0.2Sn>Fe0.1Sn>Fe3Ni>Fe1Cr.

4.2.1.10 Pitting corrosion in mild steel

Melchers (2005) investigated effects of alloying elements on the maximum depth of pits in mild steel in immersed marine environment. Results are provided in Fig. 4.7. The detailed effects were found to depend on the different corrosion phases of the bimodal model (see 2.5.11), each controlled by different mechanisms that control pitting. Some alloys appeared to be influential only in the aerobic phases (Ti, Cu, Al, Mo), others only in the anaerobic phases (Ni+Cr, Cr+Cu), and yet others in both phases (Cr+Mo, Cr+Al).

Figure 4.7 Effects of alloying elements on the pitting parameters of the bi-modal corrosion model for mild steel (based on data in Melchers, 2005a).

Blekkenhorst et al. (1986) applied extreme value distribution (Gumbel distribution) to characterize the pitting behavior of numerous steels after immersion in natural seawater (North Sea, water depth: 45–90 m). They introduced the following distribution function:

Equation (4.15)

Here, ϕ(x) is the probability of maximum pit depth, N is the total number of specimens, n is the number of the specimen with maximum pit depth=x, and y is a variable. Examples for different distributions are provided in Fig. 4.8. In the figure, T(x)=[1/(1−ϕ(x))] is the return function, and A (m²) and B (m²) are area functions. The best steel was assumed to have the largest excepted surface area required to find a pit depth>xcrit. The procedure was applied to the long-term (7.2 years) corrosion of 35 steels by Blekkenhorst et al. (1986).

Figure 4.8 Extreme values probability graphs, showing various ordinates; see Eq. (4.15). Blekkenhorst, F., Ferrari, G.M., van der Wekken, C.J., Ijsseling, F.P., 1986. Development of high strength low alloy steels for marine applications: part 1: results of long term exposure tests on commercially available and experimental steels. Br. Corr. J. 21 (3), 163–176. Reprinted by permission of Taylor & Francis Group LLC.

4.2.2 Weathering steel

4.2.2.1 General relationships

Low-carbon steels with a carbon content of less than 0.2 wt.%, alloyed mainly with copper, chromium, phosphorus, and nickel to a total alloy content of less than 3.0–4.0 wt.%, are defined as weathering steels.

Comparative corrosion rates for low-carbon steel and weathering steels after 2 years exposure in a coastal marine environment (Cabo Vilano, Spain, Atlantic Ocean) are listed in Table 4.12. It can be seen that weathering steel exhibited a lower corrosion rate in marine environments. An increase in Co (from 0.29% to 0.50%), of Ni (from 0.12% to 2.38%), and of Cr (from 0.08% to 0.46%) decreased the corrosion rates notably. The rust layers formed on weathering steel in marine environments were linearly related to the corrosion rate, and they were found to consist of two sublayers, an inner layer, and an outer layer. The outer part contained lepidocrocite and akaganeite, and the inner part consisted of ferrihydrite, maghemite, and goethite (see 2.6.11.2). Whereas Ni could be found in both corrosion product layers, Cr was concentrated in the inner layer (Cano et al., 2014); see 2.6.10.5.

Table 4.12

aGravimetrically estimated.

Effects of alloying elements on the seawater resistance of construction steels are provided in Table 4.13. Wen et al. (2016) found that an increase in Ni reduced the corrosion rate of weathering steel under simulated marine conditions (cyclic testing; 12 minutes immersion in 1.0 wt.% NaCl; 45 minutes drying at 45°C and 60%–80% RH; total cycles: 360). The results provided in Table 4.14 revealed a shift of the corrosion potential to less negative numbers as well as a decrease in current density and an increase in corrosion resistance with an increase in Ni.

Table 4.13

Table 4.14

Kashima et al. (2013) explored the corrosion performance of chromium steels in different environments (including marine atmosphere; Miyako Island, Philippine Sea) and noted that the corrosion resistance of 1.0% chromium steel was lower than that of steel without chromium in an environment with very high concentrations of chloride ions. In contrast, the addition of chromium increased the corrosion resistance in environments with low-chloride ion concentrations (Fig. 4.9).

Figure 4.9 Corrosion loss of steels after 1-year exposure in various chloride-containing environments. Redrawn from Kashima, K., Sugae, K., Kamimura, T., Miyuki, H., Kudo, T., 2013. Effect of chromium contents on atmospheric corrosion of steel in chloride environment. J. Japan Inst. Met. Mater. 77(3), 107–113.

Cano et al. (2018) investigated effects of Cu, Cr, and Ni alloying elements on the corrosion resistance of weathering steel in coastal marine atmospheres at moderate salinities [Cabo Vilano, Spain, Atlantic Ocean; chloride deposition rates: 20–41 mg/(m² day)]. The best corrosion resistance was obtained with a steel containing 3.0 wt.% Ni. The addition of Cu (0.3 wt.%) barely increased corrosion resistance, whereas the addition of 0.5 wt.% Cr notably improved the corrosion resistance.

The corrosion of weathering steels in different marine environments is discussed in detail in 2.6.10.6 (atmosphere) and 2.7.3.6 (splash zone).

4.2.2.2 Regression equations

Albrecht and Naeemi (1984) derived a regression equation for effects of alloying elements on the corrosion loss of weathering steel in a marine atmosphere:

Equation

(4.16)

In the equation, the corrosion loss C is given in mils, and the alloying elements are given in %.

4.3 Stainless steel

4.3.1 Selection

4.3.1.1 General relationships

Stainless steels shall be selected with respect to their corrosion resistance. BSH (2015) claims that the use of stainless steel for offshore wind power structured requires particular corrosion measures. Fig. 4.10 expresses the consequence when an unsuitable type of stainless steel is used for offshore wind power structures. In terms of weldability, only steel grades suitable for welding with guaranteed resistance to intercrystalline corrosion shall be used (GL., 2012). With respect to machinery for offshore wind power structures, DNVGL (2016a), stainless steels shall be selected with respect to their corrosion resistance. Unless agreed otherwise for individual cases, suitable steels, for example, according to EN 10088 (stainless steels) and EN 10213 (steel castings for pressure purposes), may be selected. Cast steel shall not contain more than 0.03% carbon (DNVGL, 2016a).

Figure 4.10 Atmospheric corrosion of unsuitable stainless steel at an offshore wind power structure (Muehlhan Holding GmbH, Hamburg, Germany).

Compositions of stainless steels subjected to seawater are listed in Table 4.15. A general ranking of stainless steel under different corrosive and marine environments is given in Table 4.16. With respect to pitting corrosion and crevice corrosion, duplex steels provided the best performance, whereas austenitic steels performed worse. The higher the pitting resistance equivalent number (PREN) values were, the better was the corrosion resistance (see 4.3.2.1). In a sheltered marine atmosphere, duplex steels performed best, and ferritic steels performed worst. Austenitic steels exhibited a moderate corrosion resistance.

Table 4.15

Table 4.16

Notes: Ranking: 0—bad; 1—moderate; 2—best. PREN, Pitting resistance equivalent number.

aPREN=%Cr+3.3%Mo+x%N (Mo>1.0%), x=0→ferritic steel, x=16→duplex steel.

If immersed in seawater, corrosion rates of stainless steels were found to increase linearly with an increase in nickel content (Reinhart, 1976).

For offshore wind power structures, the use of stainless steel is restricted in at least one situation (DNVGL, 2016a): Stainless steels to be used in the atmospheric zone or the splash zone should have a corrosion resistance equivalent or better than that of type AISI 316 [(EN 10088, WNr. 1.4401 (X5CrNiMo17-12-2)]. Bolts of this material exposed to sea spray but sheltered from direct rainfall have sometimes suffered corrosion attacks, and higher alloyed materials, such as type 25Cr duplex, should be considered for critical applications. Table 4.17 illustrates a coarse ranking for the use of stainless steels in marine environments.

Table 4.17

Note: Designations: 0=worst; 1=mean; 2=best.

The corrosion of stainless steels in different marine environments is discussed in detail in 2.6.10.7 (atmosphere), 2.7.3.7 (splash zone), 2.8.2.2 (tidal zone), 2.9.2.13 (immersed in seawater), and 2.11.3.3 (soil).

4.3.1.2 Material groups

With respect to the corrosion of stainless steels in seawater environment, DIN 81249-2 (2013) distinguishes three material groups, namely, FE 1 to FE 3. The distinctive feature is the free corrosion potential. The respective corrosion parameters are listed in Table 4.18. The materials in groups FE 1 and FE 2 are assumed to be passive in seawater with no measurable uniform corrosion and are subject to pitting corrosion only in waters containing chloride or bromide ions (DIN 81249-2, 2013). In marine atmospheres, the resistance of material groups FE 1 to FE 3 to pitting and crevice corrosion mainly increases with chromium, molybdenum, and nitrogen contents (DIN 81249-4, 2012).

Table 4.18

aSea atmosphere.

4.3.1.3 Corrosion resistance classes

The selection of corrosion-resistant materials can be performed by classifying different steel grades in to two steps (DIN EN 1993-1-4, 2015; see also Stranghöner et al., 2018a):

• estimation of a corrosion resistance factor (CRF).

• estimation of a corrosion resistance class (CRC).

CRCs are categorized into five classes (CRC), from very low (CRC I) to very high (CRC V). The determination of the CRCs for the special case of application can be done through the calculation of the CRF (DIN EN 1993-1-4, 2015; see also Burkert et al., 2018; Mietz and Burkert, 2017; Stranghöner et al., 2018a):

Equation (4.17)

Here, CRF is the corrosion resistance factor, F1 is the risk of exposure to chlorides, F2 is the risk of exposure to sulfur dioxide, and F3 is the cleaning regime or exposure to washing by rain. Values for F1 to F3 are listed in DIN EN 1993-1-4 (2015); see also Mietz and Burkert (2017). F1-values for stainless steels in chloride-containing marine environments are provided in Table 4.19, F2-values are provided in Table 4.20, and F3-values for three cleaning regimes are listed in Table 4.21. Based on CRF-values, the following five (I–V) CRCs are defined (DIN EN 1993-1-4, 2015):

CRF=1I

0≥CRF>−7II

−7≥CRF>−15III

−15≥CRF≥−20IV

CRF<−20V

Table 4.19

Table 4.20

aAverage gas concentration.

bUsually valid for European coastal environment.

Table 4.21

Stainless steels suitable for different CRCs are listed in Table 4.22. Examples for corrosion class assessments for duplex steels for a number of European near-shore bridge constructions are provided in Mameng et al. (2019).

Table 4.22

Note: CRC, Corrosion resistance class.

Mameng et al. (2017) investigated the corrosion resistance of 11 stainless steels (ferritic, austenitic, duplex) in a severe atmospheric environment (Dubai, Persian Gulf; corrosivity: CX) and found the corrosion resistance of the steels to well agree with the CRCs system according to DIN EN 1993-1-4 (2015).

4.3.2 Pitting corrosion resistance

4.3.2.1 Pitting resistance equivalent number

The PREN is an indication of the resistance of stainless steels and nickel-based alloys to pitting corrosion and crevice corrosion in the presence of chloride-containing water. It can be estimated as follows (DIN 81249-1, 2011). For austenitic stainless steels with ≥3.0 wt.% molybdenum as well as for nickel-based alloys:

Equation (4.18)

For austenitic-ferritic stainless steels:

Equation (4.19)

For austenitic steels with <3.0 wt.% molybdenum as well as for the austenitic-ferritic steel 1.4460:

Equation (4.20)

For stainless steels and nickel-based alloys, ISO 8044 (2015) suggests the following relationship:

Equation

(4.21)

In the equations, PREN is the pitting resistance equivalent number, Cr is chromite, Mo is molybdenum, W is tungsten, and N is nitrogen. At comparable Cr and Mo contents, the nitrogen content increases the pitting corrosion resistance (DIN 81249-1, 2011). Burkert and Lehmann (2015) highlighted the fact that PREN is basically a theoretically calculated number, and that practical PREN numbers can become substantially lower. An example presented by the authors was the precipitation of an intermetallic σ-phase in the surface zone of a stainless steel (1.4462) in North Sea service, which reduced the practical PREN to <18 (compared to the theoretically calculated PREN=32).

The higher the PREN, the higher is the resistance against pitting corrosion. Pedeferri (2018) claimed that stainless steel with PREN numbers higher than 35–40 could resist pitting in seawater, provided there are no galvanic couplings with carbonaceous materials. For best performance, even in the presence of chlorine, PREN>45 was considered mandatory (Pedeferri, 2018). Table 4.23 lists PREN numbers for stainless steels considerable for chloride-containing waters. In the table, the stainless steels are categorized into four groups, namely, Group 1 (PREN 16–20), Group 2 (PREN 25–27), Group 3 (PREN 34–38), and Group 4 (PREN 41–54).

Table 4.23

aPREN=%Cr+3.3%Mo+x%N; average values, rounded up.

Results of an early investigation into the long-term (8 years) corrosion resistance of stainless steels immersed in tropical seawater (Panama Canal zone, Pacific Ocean) could not fully prove this assumption for pitting corrosion; however, the amounts of chromium (17% vs 18%) were very close (see Alexander et al., 1960). More recent long-term site exposure investigations on stainless steels under offshore conditions (Helgoland, North Sea) have shown that pitting depth reduced with an increase in PREN (Burkert and Lehmann, 2015; Burkert et al., 2017, 2018). The investigated materials are characterized in Tables 4.24 and 4.25. The authors measured pit depth (µm) and extent of surface corrosion (with a scheme between 1 and 10, whereby a higher number corresponded to less surface corrosion). Regarding the steel types, the ferritic steel, alloyed with molybdenum (1.4521), and the duplex steel (1.4062) performed best with respect to the surface quality. The ferritic steel 1.4003 showed an inconsistent corrosion resistance and featured severe pitting. In general, the authors noted a superior performance of the duplex steels, which was attributed to the elevated amounts of chromium and nitrogen. This may have led to better passivation and, in turn, to a higher resistance against pitting. Because chromium and nitrogen are introduced into Eq. (4.18), PREN increases if these elements are alloyed into the steel.

Table 4.24

aPREN=%Cr+3.3%Mo+x%N (Mo>1.0%), x=0→ferritic steel, x=16→duplex steel.

Table 4.25

Results of pit depth measurements are plotted against calculated PREN numbers in Fig. 4.11. The results revealed a stabilization of the pit formation kinetics for PREN≤24. If this value was exceeded, pit depth started to decrease exponentially. Fig. 4.12 shows pit depths for two exposure periods, namely, 22 and 60 months. Interestingly, pitting depth did not increase necessarily for the longer exposure times. This effect was most pronounced for the duplex steels.

Figure 4.11 Effect of PREN on pit depth in stainless steels after 22 months exposure to a severe marine environment. PREN, Pitting resistance equivalent number. Redrawn from Burkert, A., Müller, T., Lehmann, J., 2017. Langzeitkorrosionsverhalten nichtrostender Stähle in maritimer Atmosphäre. in: Korrosionsschutz in der maritimen Technik, DNVGL, Hamburg, Germany, 2017, 7–19.

Figure 4.12 Exposure time effect on pit depth in stainless steels exposed to a severe marine atmosphere. (A) Mean pit depth after 22 months. (B) Mean pit depth after 60 months. Burkert, A., Müller, T., Lehmann, J., Mietz, J., 2018. Long-term corrosion behaviour of stainless steels in marine atmosphere. Mater. Corr. 69, 20–28. Reprinted by permission of Wiley and Sons, Inc.

Al-Fozan and Malik (2005) investigated the effect of PREN on pit depth in different steels (carbon steel, austenitic stainless steels, Cu-based alloys, Ni-based alloys) in different marine environments (laboratory immersion tests; above sea level, semi-immerged, immerged) for an exposure time of 450 days. Mameng and Wegrelius (2019) explored the long-term (6.5 years) corrosion performance of numerous stainless steels in a coastal marine atmosphere (Bohus, Sweden, Kattegat; corrosivity: C5 to CX). Both references used the following PREN definition (DIN 81249-1, 2011; Norsok, 2014); see Eq. (4.19):

Equation (4.22)

Al-Fozan and Malik (2005) found that pitting was suppressed for PREN numbers higher than 50, irrespectively of the marine environment (above sea level, semi-submerged, submerged). This result is verified in Fig. 4.13 for a marine atmospheric environment. The dynamics of the pit depth formation, however, was different in the three atmospheres (see Table 4.26). The pit depth decreased almost linear with increasing PREN above sea level. Under semi-immerged and immerged conditions, the pit depth formation was accelerated at PREN numbers in excess of 25. It was also found that pit formation was reduced for Mo contents larger than about 2.5%, and that Mo contents of 9% eliminated pitting in all marine environments (Al-Fozan and Malik, 2005). Hoffmeister (2001) found a relationship between a reduced PREN number and the pitting potential of stainless steels in natural seawater (Fig. 2.370). The results can be fitted with a second order polynomial regression (R²=0.996):

Equation (4.23)

Here, PPit is the pitting corrosion potential in mVSCE, and PRENR is a reduced PREN-parameter (%Cr+3.3%Mo). The pitting resistance of stainless steel in seawater can also be characterized through a resistance index. This index is a function of the pitting potential of the steel (Cunat, 2008). Pitting resistance index values for a number of stainless steels are listed in Table 4.27.

Figure 4.13 The effect of PREN on maximum pit depth in stainless steel after 4 years exposure in a marine environment (corrosivity CX; chloride deposition: 126 mg/m²). PREN, Pitting resistance equivalent number. Redrawn from Mameng, S.H., Wegrelius, L., Pettersson, R., Leygraf, C., 2015. Atmospheric corrosion resistance of stainless steel: results of a field exposure program in the Middle-East. Acom 3, 2–12.

Table 4.26

Table 4.27

aDepends on pitting potential.

The results presented in Fig. 4.13 were approximated with a linear regression (Mameng et al., 2015, 2016):

Equation (4.24)

Here, hPmax is the maximum pit depth in µm. The regression coefficient was R²=0.71. The reverse linear relationship was also found by Burkert et al. (2018) for stainless steels after 5 years in an offshore environment (Helgoland, North Sea) and by Mameng and Wegrelius (2019) for stainless steels after 6.5 years in a coastal marine atmosphere (Bohus, West Sweden, Kattegat; corrosivity: C5 to CX). The latter authors derived the following relationships. For open conditions (R²=0.80):

Equation (4.25)

For sheltered conditions (R²=0.73):

Equation (4.26)

In the equations, hPmax is the maximum pit depth in µm, and PREN is calculated with Eq. (4.22). The authors also introduced a rating factor for corroded stainless steels, ranging between RN1 and RN9, whereas the latter rating characterized 100% rust. The relationships to the PREN were found to follow reverse linear functions. For open conditions (R²=0.91):

Equation (4.27)

For sheltered conditions (R²=0.80):

Equation (4.28)

In the equations, RN is the rating factor, and PREN is calculated with Eq. (4.22). Mameng et al. (2017) investigated the corrosion performance of 12 stainless steels in a marine atmosphere (Dubai, Persian Gulf). To assess the materials, they subdivided the PREN of the stainless steels into bulk material PREN according to Eq. (4.22) and a modified surface layer PRENS:

Equation (4.29)

For the long-term (4 years) exposure of the stainless steels (ferritic, austenitic, duplex) under severe marine atmospheric conditions (Dubai, Persian Gulf; corrosivity: CX), Mameng et al. (2017) found the following regression functions between the rating factor and the surface PRENS. For open conditions (R²=0.63):

Equation (4.30)

For sheltered conditions (R²=0.48):

Equation (4.31)

In the equations, RN is the rating factor, and PRENS is calculated with Eq. (4.29). With respect to the bulk material PREN, Mameng et al. (2017) developed the following regression equations. For open conditions (R²=0.92):

Equation (4.32)

For sheltered conditions (R²=0.91):

Equation (4.33)

In the equations, RN is the rating factor, and PREN is calculated with Eq. (4.22). The progress of the decreasing regression functions was notably higher for the surface PRENS compared with the bulk material PREN, but the regression coefficients were low for the surface PRENS. Mameng et al. (2017) found that the PREN of the bulk material of stainless steels according to Eq. (4.22) provided the best correlation to pitting corrosion resistance. They also claimed that stainless steels with PREN≥35 were suitable for the application in severe marine environments if combined with frequent cleaning.

Values listed in Table 4.28 illustrate the relationship between PREN and critical pitting temperature for stainless steels in seawater. Craig (2021) and Gunn (2003) reported linear relationships between the critical pitting temperature and PREN according to Eq. (4.19) for a number of stainless steels in natural seawaters.

Table 4.28

Machuca et al. (2012a) investigated the resistance of stainless steels to localized corrosion in natural seawater (Rottnest Island, Western Australia). Martensitic (13%Cr) stainless steel and austenitic (316L) stainless steel were found to be very susceptible to seawater corrosion at temperatures of 20°C and 30°C. The localized corrosion of these steels was exacerbated by prolonged exposure to seawater and by the increase in seawater temperature. Microorganisms in seawater further accelerated the localized corrosion processes. Duplex (2205) stainless steel exhibited good resistance to localized corrosion in seawater at 20°C–30°C, regardless of the exposure time and of the presence of microorganisms in the seawater. The stainless steels were ranked in terms of their resistance to localized corrosion and microbiologically influenced corrosion in natural seawater at 20°C and 30°C as follows: duplex (2205 DSS)>austenitic (316L SS)>martensitic (13%Cr SS).

Results listed in Table 4.29 indicated that the PREN according to Eq. (4.22) is a useful guide to rank the alloys resistance to localized corrosion in natural seawater (Rottnest Island, Western Australia). However, the results also suggested that effects of microstructure and its interaction with the additions of specific alloying elements should be considered when selecting the correct PREN formula to compare the localized corrosion resistance of different steel grades. When considering duplex stainless steels, the use of the PREN number formula with 30% instead of 16% for nitrogen (PREN = 1.0·%Cr+3.3·%Mo+30·%N) was recommended (Machuca et al., 2012b). For samples exposed to a coastal marine atmosphere (Kure Beach, United States; Atlantic Ocean), Brown et al. (1964) found the following ranking for the pitting sensitivity of stainless steels: 410>347>321>304>316.

Table 4.29

Note: PREN, Pitting resistance equivalent number.

aPREN=%Cr+3.3%Mo+x%N (Mo>1.0%), x=0→ferritic steel, x=16→duplex steel.

The PREN was found to affect the pitting induction time (Pedeferri, 2018):

Equation (4.34)

Here, ti is the pitting induction time in hours, Cl is the chloride concentration in %, ΔEpit is the potential difference in V, and k is a factor (~1.0 hour). Other localized corrosion parameters which depend on PREN include critical pitting temperature (2.15.2.4), critical chloride concentration (2.15.2.4), and critical gap size (2.16.2).

Craig (2021) claimed that the application of the standard equations for PREN is strictly only applicable to single-phase austenitic stainless steels in aerated or oxidizing environments, and that the use of these equations beyond these narrow boundaries is not fundamentally sound. For multiphase alloys, such as duplex stainless steels and nickel-based precipitation hardening alloys, any PREN formulae must be described and bounded by their processing history, phase balances, and microstructure (Craig, 2021).

4.3.2.2 Critical pitting index

Steinsmo et al. (1997) utilized an index number almost equal to that given in Eq. (4.22). It is denoted critical pitting index and is defined as follows:

Equation (4.35)

Here, CPI is the critical pitting index. The authors found that this index number was linearly related to the critical pitting temperature:

Equation (4.36)

Here, CPT is the critical pitting temperature in °C, and C0 is a linear constant. Respective results are presented in Fig. 4.14. Oldfield (1987) found a linear relationship between molybdenum content in stainless steels and nickel-based alloys and critical pitting temperature.

Figure 4.14 CPT as function of CPI for stainless steels in chlorinated seawater; see Eq. (4.36). CPI, Critical pitting index; CPT, critical pitting temperature. Trends are based on data in Steinsmo et al. (1997).

4.3.2.3 Alloying resistance number

For stainless steels, Speidel (2006), Speidel and Cui (2003), and Speidel and Speidel (2004) introduced a parameter alternatively to the PREN. This parameter MARC: Measure of Alloying for Resistance against Corrosion was defined as follows (Speidel, 2006; Speidel and Cui, 2003):

Equation

(4.37)

In the equation, the alloying elements are given in %. In contrast to PREN, the MARC value considered elements that are not beneficial to the corrosion resistance, namely, Mn and Ni. Relationships between MARC and critical corrosion temperatures are shown in Fig. 4.15. Speidel and Speidel (2004) also noted relationships between MARC and stress corrosion crack growth rates in austenitic and duplex stainless steels.

Figure 4.15 Relationships between alloy composition of stainless steels and critical corrosion temperatures. Speidel, H.J., Speidel, M.O., 2004. Nickel and chromium-based high nitrogen alloys. Mater. Manufact. Proc. 19 (1), 95–109. Reprinted by permission of Francis & Taylor Group LLC.

4.3.2.4 Effects of microstructure

Reilhac et al. (2019) addressed effects of the microstructure on the corrosion of martensitic steels in substitute seawater (ASTM D1141). PREN was found to be a good index for a first classification of martensitic stainless steels, but other characteristics that are rarely present in alloy description (localization of chromium carbide, dislocation density) and parameters (internal stress, degree of sensitization) could provide a better distinction between the steels. Results listed in Table 4.30 indicated that chromium carbide seemed to be the most influencing parameter for the corrosion behavior as they can limit passive layer growth and introduce intergranular corrosion sensitivity. In alloys with a higher homogeneity in terms of microstructure and alloying elements, dislocation density appeared to be the main limiting factor of corrosion resistance.

Table 4.30