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Chloride stress corrosion cracking in austentic stainless steel – recommendations for assessing risk, structural integrity and NDE based on practical cases and a review of literature

ES/MM/09/48

Lead Authors: R Parrott BSc PhD MIMMM CEng H Pitts MEng PhD

DISTRIBUTION Mr R Breen HSE Principal Specialist Inspector, Mechanical Engineering Mr G Hughes HSE Specialist Inspector, Mechanical Engineering Mr S Pointer HSE Principal Specialist Inspector, Mechanical Engineering Dr A Curran Director, HSL Dr P Bridges Head of Engineering Safety Unit, HSL Dr W Geary Technical Lead for Metallurgy and Materials, HSL Mr P Heyes Manager - Incident investigations

Dr J Hobbs HSL Dr H Pitts HSL Dr Parrott HSL LIS File

PRIVACY MARKING:

Available to the public.

Report Authorised for Issue by: Mr P Heyes Date of issue: July 2010 Project Manager: Dr R Parrott Contributing Authors: Dr H Pitts Technical Reviewer(s): Dr W Geary, Mr P Heyes Editorial Reviewer: Dr P Bridges HSL Project Number: JN0004220

© Crown copyright (2010)

ACKNOWLEDGEMENTS The authors wish to acknowledge the assistance of:

(i) The chemical manufacturing company that provided detailed background information on the operation of two stainless steel reactors that developed chloride stress corrosion cracking.

(ii) The company’s insurer for supplying details of a metallurgical investigation carried out on the reactors.

(iii) Mitsui-Babcock who carried out an assessment of NDE techniques on samples from the reactors.

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CONTENTS

1 INTRODUCTION......................................................................................... 1 1.1 Background ............................................................................................. 1 1.2 Austenitic stainless steel.......................................................................... 2 1.3 Cl-SCC mechanism ................................................................................. 3 1.4 Factors affecting Cl-SCC ......................................................................... 4 1.5 Controlling Cl-SCC .................................................................................. 5

2 RECOMMENDATIONS............................................................................... 6 2.1 Risk assessment for Cl-SCC ................................................................... 6 2.2 Structural integrity assessment.............................................................. 10 2.3 Non-destructive examination ................................................................. 11

3 CRACKING OF THE REACTOR VESSELS............................................. 13 3.1 History ................................................................................................... 13 3.2 Discussion of Cl-SCC in the reactors..................................................... 14

4 AN ASSESSMENT OF NDE TECHNIQUES FOR CL-SCC ..................... 16 4.1 Background ........................................................................................... 16 4.2 Conclusions from the NDE Assessment ................................................ 16 4.3 Overview of NDE issues for Cl-SCC...................................................... 16

5 LITERATURE REVIEW ............................................................................ 17 5.1 Current understanding of the Cl-SCC mechanism................................. 17 5.2 Practical cases of Cl-SCC below 600C .................................................. 20 5.3 Effect of testing technique ..................................................................... 24 5.4 Environmental factors ............................................................................ 25 5.5 Other factors.......................................................................................... 30 5.6 Metallurgical factors............................................................................... 31

6 CONCLUSIONS........................................................................................ 35 6.1 Cl-SCC in the Reactors ......................................................................... 35 6.2 From the literature review ...................................................................... 35

7 APPENDICES........................................................................................... 37 7.1 Appendix 1 – Metallurgical examination ................................................ 37 7.2 Appendix 2 - Engineering assessment .................................................. 39

8 REFERENCES.......................................................................................... 43

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EXECUTIVE SUMMARY Objectives

Chloride stress corrosion cracking (Cl-SCC) is one of the most common reasons why austenitic stainless steel pipework and vessels deteriorate in the chemical processing and petrochemical industries. The objectives of this work were:

To draw conclusions and give recommendations for best practice in assessing the risk of Cl- SCC and in applying risk based inspection (RBI) to existing plant, in particular setting inspection intervals and carrying out non-destructive examinations (NDE).

To assess a case of extensive deterioration from Cl-SCC in austenitic stainless steel reactor vessels that operated at ambient temperature. This part of the work included metallurgical testing and an engineering critical assessment of the reactors’ structural integrity.

To review NDE techniques for detecting and sizing Cl-SCC based on trials carried out by Mitsui-Babcock with samples from one of the reactors.

To review literature on published cases of Cl-SCC at near ambient temperatures and of factors affecting the mechanism of Cl-SCC. The purpose of the review was to assess published data as a basis for control measures and for RBI decisions in the management of Cl-SCC.

Recommendations for Risk Assessment of Cl-SCC

The risk of Cl-SCC is usually assessed on the basis of chloride content, pH and temperature. In our view there are additional factors that should be taken into account when assessing the risk with both new and existing pipework or vessels that have accumulated significant service. These include:

• Operation involves high temperature excursions. Risk should be determined by the highest temperature reached during any part of duty or maintenance operations, irrespective of the duration of the excursion.

• Liquid can dry out allowing chlorides to concentrate or form chloride-rich solid films.

• Pitting and/or crevice corrosion already exist.

• The steel was manufactured before 1970 with possible higher levels of impurities.

• Possibility of sensitisation.

• Free machining grades.

• Surface finish has deteriorated since manufacture.

• Iron contamination of surfaces.

• Welding during manufacture, modification and repair.

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• Design or manufacturing details where chlorides can accumulate, e.g. roots of partial penetration welds.

• Operation causes stress cycles.

It is recommended that the risk of Cl-SCC is assessed by extending the API 581 approach to take account of these factors. A flow diagram for the proposed extension of the API 581 method, and crack growth rates are suggested for risk categories. These proposals are outlined in Section 2 of this report, in particular Figure 2, Table 2, Table 3 and Table 4.

Recommendations for Structural Integrity and NDE

Wrought austenitic stainless steels have high fracture toughness and for pipework and vessels Leak-Before-Break is the most likely consequence of Cl-SCC. Leak detection is not a reliable indicator of Cl-SCC because cracks are highly branched and may be filled with corrosion products. Nevertheless, it is recommended that where pipework or vessels develop leaks in service, they should always be investigated for possible Cl-SCC by NDE or by in-situ metallography.

Cl-SCC can generate very large cracks in structures where, as in the case of reactors, the residual stress from welding dominates and operational stresses are low by comparison. If undetected by NDE, the large cracks might introduce failure modes with consequences that were not anticipated by the original design, e.g. complete separation of attachments, toppling of tall columns under wind loading or collapse of long pipe runs due to self-weight.

The simplest and most effective NDE technique for detecting Cl-SCC is dye penetrant testing. Eddy Current Testing (ECT) is effective with purpose-designed probes that have been calibrated on known defects. ECT was found to be ineffective on the samples from the reactor due to limited penetration and sensitivity to surface imperfections that could not be distinguished from cracking.

Crack sizing by eddy current testing may be limited and is not possible by penetrant testing.

Ultrasonic flaw detection can be applied as a manual or an automated NDE technique for detecting Cl-SCC. For structures with complex design features and welds as on the reactors, the trials indicated that ultrasonic testing would require a range of probes, several complimentary scans and be very time consuming. Ultrasonic flaw detection did not cover all design details and possible crack position orientations found on the reactor, and crack sizing was difficult.

Main findings on Cl-SCC in the reactor

It is likely that the following factors contributed to Cl-SCC in the reactor:

• Residual stress from fabrication and welding

• A rough surface finish leading to a long period of slow localised corrosion.

• Initiation of Cl-SCC on the process side when the depth of corrosion pitting was ~1mm.

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• Short periods of rapid crack growth when the temperature ≥600C during the cleaning cycles.

• Long periods of very slow crack growth at the normal reactor operating temperature.

• Periodic reactivation of cracking due to low frequency load cycles and the temperature excursions during cleaning.

From the Literature Review

Cl-SCC initiates from sites of localised pitting or crevice corrosion. Cl-SCC propagation occurs when cracks grow more quickly from the pit or crevice than the rate of corrosion.

The initiation of Cl-SCC has been shown to involve a competition between localised corrosion, which is strongly dependent on chloride concentration but has a weak dependence on temperature, and crack growth which has a strong dependence on temperature but is relatively unaffected by chloride concentration and pH.

It follows from the competition approach that environmental factors, which affect localised corrosion, are also likely to affect the initiation of Cl-SCC. Furthermore, it also follows that more severe conditions will be required to initiate Cl-SCC than are needed to sustain crack growth. Recent work has clearly shown that Cl-SCC crack growth can be sustained at a chloride concentration and temperature significantly below those required to initiate cracking.

There is a large amount of published work on various aspects of Cl-SCC in austenitic stainless steels. However, no data were found that could be used to predict the time required for crack initiation by localised corrosion in real structures.

Fracture mechanics tests have shown that Cl-SCC propagation can begin at low stress intensities in the range 2MPa.m0.5 to 10MPa.m0.5. For fabricated structures containing tensile residual stresses, the critical depth of localised corrosion to initiate Cl-SCC would be <1mm.

The rate of crack propagation is strongly dependent on temperature but is relatively unaffected by stress intensity. Rates of Cl-SCC propagation can vary from 0.6mm.yr-1 at near ambient temperatures to >30mm.yr-1 at temperatures ~1000C. In laboratory tests Cl-SCC has been observed in samples at temperatures between 250C and 400C.

The majority of the reported practical instances of Cl-SCC have occurred where temperatures ≥600C. However, a significant number of failures below 600C have also been reported although in these instances there appear to have been other contributory factors which include:

• The use of highly cold worked and/or free-machining grades.

• Iron contamination of the surface.

• The presence of a highly corrosive film containing chloride compounds.

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1 INTRODUCTION

1.1 BACKGROUND

Chloride stress corrosion cracking (Cl-SCC) is one of the most common reasons why austenitic stainless steel pipework and vessels deteriorate in the chemical processing and petrochemical industries. Deterioration by Cl-SCC can lead to failures that have the potential to release stored energy and/or hazardous substances. Failures of plant can be prevented by an awareness of the onset and evolution of Cl-SCC, and by periodic inspection to monitor the extent of cracking. Although the deterioration of austenitic stainless steels by Cl-SCC is well known, recent incidents and inspection visits by HSE have found that risk assessments were inconsistent and did not always take account of current knowledge. Discussions between HSE, dutyholders and competent bodies identified that the technical justification for setting inspection intervals and the effectiveness of periodic non-destructive examination (NDE) for monitoring Cl-SCC were additional areas of concern.

This report describes work carried out for Mr R Breen, Principal Inspector for Mechanical Engineering in the Hazardous Industries Directorate of the Health and Safety Inspectorate. The report is in five parts:

• Section 1 is an introduction to Cl-SCC in austenitic stainless steel.

• Section 2 gives the authors’ conclusions and recommendations for assessing the risk of Cl-SCC. These include applying RBI, setting inspection intervals, structural integrity considerations and NDE.

• Section 3 summarises findings from a metallurgical investigation into extensive deterioration from (Cl-SCC) in austenitic stainless steel reactor vessels that had been operated at ambient temperature. This includes an engineering assessment of the reactor structural integrity. The results from the laboratory examination are in Appendix 1 and the engineering assessment is in Appendix 2.

• Section 4 is a short review of non-destructive testing techniques for detecting Cl-SCC in thin-walled austenitic stainless steel vessels with complex design and welded details. This includes the results of NDE trials carried out by Mitsui-Babcock on samples from two of the failed reactors.

• Section 5 is a literature review of published cases of Cl-SCC at near ambient temperatures and of factors that affect the mechanism. The purpose of the review was to assess published data as a basis for control measures and risk based inspection (RBI) decisions in the management of Cl-SCC.

Austenitic stainless steel pipework and vessels are particularly vulnerable to Cl-SCC if they are covered with an insulation material that contains moisture, i.e. conditions that normally cause corrosion under insulation (CUI) of carbon and low alloy steels. This report is primarily concerned with Cl-SCC from the process environment and from the outside due to the external environment where no insulation material is involved. Nevertheless, some comparisons will be made with Cl-SCC of insulated and un-insulated austenitic stainless steel.

1.2 AUSTENITIC STAINLESS STEEL

Austenitic stainless steels are iron-based alloys that contain nominally 19% chromium and 9% nickel. As implied by the name, austenite is the predominant microstructural phase in austenitic stainless steels at room temperature. The chemical composition can be varied, for example by lowering the carbon content, and by adding titanium, niobium or tantalum to prevent carbide formation1, or by adding molybdenum to increase resistance to localised corrosion. Table 1 lists the grades of austenitic stainless steels that are most widely used in chemical processing. Table 1 is not an exhaustive list and the grade designations are closest matches based on chemical composition rather than exact equivalents. For the purposes of this report, the American Iron and Steel Institute numbering system has been used when referring generally to a grade.

AISI Euronorm DIN British Description Standard 304 1.4301 X5CrNi18-10 304S31 The general-purpose grade, widely used where good formability and corrosion resistance are required. 304L 1.4306 X2CrNi19-11 304S11 As 304 but with lower carbon content to minimise carbide precipitation during welding. 301 & 1.4310 X12CrNi18-10 301S21 Higher strength versions of 304 that are 302 often cold worked to give higher strength. 303 & 1.4305 X10CrNiS18-9 303S31 General purpose grades with sulphur or 303Se selenium added to improve machinability 321 1.4541 X6CrNiTi18-10 321S12 As 304 with an addition of titanium to 321S31 prevent carbide precipitation during welding. 347 1.4450 X6CrNiNb18-10 347S31 As 304 with addition of niobium and or tantalum to prevent carbide precipitation during welding. 316 1.4401 X5CrNiMo17-12-2 316S31 As 304 but with molybdenum added to increase resistance to localised corrosion in marine and chemical environments. 316L 1.4404 X2CrNiMo17-13-2 316S11 As 316 but with lower carbon content to minimise carbide precipitation during welding.

Table 1. Grades of austenitic stainless steel most widely used in chemical plant.

1 The formation of chromium carbide or other complex carbides at grain boundaries in austenitic stainless steels causes a loss of corrosion resistance and increased susceptibility to intergranular corrosion. Carbide are formed when austenitic stainless is exposed to temperatures between 4500C and 8000C. This phenomenon is known as ‘sensitisation’. Further information on sensitisation is in Section 5.6.3.

1.3 CL-SCC MECHANISM

The mechanism of Cl-SCC is complex and the current understanding is discussed in Section 5 of this report. Essentially Cl-SCC involves a combination of the electrochemistry of metal dissolving over a highly localised area, i.e. at the base of a pit or crevice, and microstructural processes that separate the metal structure in a region of highly localised plastic strain, i.e. at the crack tip. A detailed review of candidate mechanisms is set out in the literature survey. For the purposes of this report, the mechanism will be described simply in terms of an initiation stage, dominated by electrochemical mechanisms, and a crack propagation stage in which both electrochemistry and metal separation are involved.

The high corrosion resistance of austenitic stainless steels in most atmospheric and aqueous environments is due to passivation by a thin (~2nm) layer of chromium oxide [2]. Wet and humid environments containing chloride ions can cause pitting corrosion and crevice corrosion of austenitic stainless steel components. Components under an applied or residual stress can deteriorate further by stress corrosion cracking in these conditions. Pitting is simply a breakdown of the chromium oxide layer followed by localised corrosion that produces pits, which may cause perforation of a vessel or pipework. Pitting is mainly associated with microscopic heterogeneities in a surface rather than macroscopic physical features of a component. Crevice corrosion is also a breakdown of the chromium oxide layer followed by localised corrosion but in contrast to pitting, it occurs at specific physical features where a surface is partly shielded and stagnant solution exists at an interface with the shielded area.

Where pitting or crevice corrosion causes localised metal loss, the geometry and the local environment are critical in ensuring that metal loss becomes self-sustaining. Localised corrosion in an active pit or inside a crevice produces a solution with the following characteristics.

(i) Higher chloride level than the bulk solution because negatively charged chloride ions migrate into the pit to balance the positively charged metal ions.

(ii) Strongly acidic with a very low pH (~0).

(iii) Nearly saturated with complex ions produced from dissolved metal, chloride and water.

This demonstrates an essential feature of localised corrosion on stainless steels, i.e. the region inside a pit, a crevice or a crack is an isolated electrochemical cell that contains a much more aggressive environment than the bulk solution.

It is commonly accepted that Cl-SCC initiates from sites of active pitting or crevice corrosion [2] and therefore, cracks are considered to grow in the high chloride, strongly acidic, near- saturated solution that develops at sites of localised corrosion. One theory [43] proposes that Cl-SCC only occurs when a crack grows more quickly than the rate of metal removal by localised corrosion from the base of a crevice or pit; in other words there is a competition between the rate of Cl-SCC and the rate of localised corrosion. Crack growth is also restricted to a range of electrochemical potential which is defined by an upper limit where dissolution exceeds crack growth and a lower limit set by re-passivation. This approach has been used by Tsujikawa to demonstrate a temperature dependency of Cl-SCC because crack growth increases more rapidly with temperature than the rate of localised corrosion. Figure 1 is a schematic description of Tsujkawa’s theory. In our opinion, the competition theory reinforces the fact that localised corrosion is a prerequisite for Cl-SCC propagation in austenitic stainless steels. Stress corrosion tests carried out by Tsujikawa and recent tests carried at HSL [77] have

confirmed that it is very difficult to initiate Cl-SCC on bare, smooth specimens under laboratory conditions when there is no localised corrosion of the surface under stress.

The susceptibility of austenitic stainless steels to Cl-SCC depends on a range of environmental variables that include chloride concentration, temperature and pH. Other variables include, for