Stainless steels limits of performance

Harpur Hill, Buxton Derbyshire, SK17 9JN T: +44 (0)1298 218000 F: +44 (0)1298 218590 W: www.hsl.gov.uk

A review of performance limits of stainless steels for the offshore industry.

ES/MM/10/11

Project Leader: E A Geary PhD

Author(s): E A Geary PhD

Science Group: Engineering Safety

DISTRIBUTION Mr A Duncan HSE, OSD Customer Project Officer Mr W Jones HSE, OSD Mr R Patel HSE, OSD Mr S Pointer HSE, CI Mr H Bainbridge HSE, ND Mr I Thomas HSE, ND

Mr P F Heyes HSL, Investigations Manager Dr A D Curran HSL, Director, Science and Resources Dr P A Bridges HSL, Unit Head, Engineering Safety Dr E A Geary HSL, Engineering Safety Dr W Geary HSL, Technical Lead, Engineering Safety

Archive File

PRIVACY MARKING:

Not to be communicated outside HSE without the approval of the authorising officer:Mr A Duncan

HSL report approval: P F Heyes Date of issue: 20th January 2011 Job number: JN0004773 Registry file: 028947 Electronic file name: JN0004473 Final report and Review

© Crown copyright (2010)

CONTENTS

1 INTRODUCTION......................................................................................... 1

2 THE APPROACH TAKEN TO FORMAT THE DATA FROM THE LITERATURE..................................................................................................... 2 2.1 The range of limits available for austenitic grade 316.............................. 3

3 A COMPARISON OF TEMPERATURE LIMITS WITHIN OFFSHORE OPERATOR GUIDELINES WITH THOSE AVAILABLE FROM THE LITERATURE................................................................................................... 10 3.1 The approach taken by the operators. ................................................... 10 3.2 Typical chemistries for stainless steel alloys. ........................................ 11 3.3 Austenitic Steels .................................................................................... 13 3.4 Super Austenitic Steels.......................................................................... 14 3.5 Duplex Steels ........................................................................................ 16 3.6 Super Duplex Steels .............................................................................. 18 3.7 Other Stainless Types ........................................................................... 19 3.8 Summary of data ................................................................................... 19 3.9 Observations ......................................................................................... 23 3.10 Concluding remarks............................................................................... 24

4 APPENDICES........................................................................................... 26 4.1 Appendix 1 - Field terms used in the spreadsheet................................ 26 4.2 Appendix 2 - Sources of data .............................................................. 27 4.3 Appendix 3 - References ..................................................................... 30

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EXECUTIVE SUMMARY Objectives

To evaluate the critical temperatures for chloride stress corrosion cracking, pitting corrosion and crevice corrosion for the range of stainless alloys currently used in topside operations in the offshore oil & gas industry. This has taken the form of a search of public domain literature and the acquisition of operational limits from a selection of operators.

Main Findings

Owing to the broad remit of the work, and its ongoing nature, it has proved difficult to tabulate simply and concisely all the literature data into useable guidelines on temperature limits. In part, this is because of the large number of variables involved and the relationship between the variables being complex. An added difficulty is that data obtained in the laboratory cannot necessarily provide temperature limits for actual service conditions. Presenting the data in a concise tabular format has not yet been practicable.

In the light of these difficulties, the approach taken has been to extract data regarding critical temperature limits from the literature and incorporate them into a searchable database currently in the form of an Excel Spreadsheet. The spreadsheet can be queried to isolate data of interest. Notes in the last column of each record, in addition to the data, provide the necessary detail from which an assessment of a specific alloy’s suitability for use can be made. It is the author’s view that the database will provide a valuable repository of corrosion-related service failure data, in addition to laboratory derived data, from which records pertaining to particular alloys, environments, and usages will be extracted. The data will ultimately provide a valuable pan- sector resource that will contain the latest academic data regarding critical temperatures but, possibly of more value to HSE, recent and salient data surrounding individual in-service corrosion failures.

Resource limits have resulted in only 100 of the 210 references obtained from literature in the public domain, which contain critical temperature data, being assessed for inclusion into the ongoing database thus far. Although this has generated over 500 individual records, it is clear that the database is far from complete and certainly less complete than is desirable. As such, this report should be regarded as the first of perhaps several instalments to be issued as the database is updated.

An additional piece of work has been carried out by the author to compare the database with limits currently prescribed by offshore oil and gas operators, as well as International and National Standards: the concern being that these latter data do not reflect the latest findings of corrosion research regarding temperature limits. Where there are discrepancies, these have been highlighted and recommendations for changes to limits have been made as follows:

1. Inconsistencies regarding operational temperature limits within the offshore sector had been identified as long ago as 2003, in that instance, for the use of duplex grades [1]. The present work has indicated that this inconsistency still exists and that it extends across the full range of stainless steels used offshore. There is, therefore, still a need for a review and revision of the temperature limits to reflect the latest developments in corrosion research.

2. The variability of temperature limits used by industry is greatest for chloride stress corrosion cracking (CSCC) and this may be the best initial focus for an exercise to

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rationalise and review guidelines. There appears to be greater consistency in the pitting and crevice corrosion limits, with the proviso that there is significantly less data from the operators compared with SCC.

3. Although it is reasonable to assume that a starting point for review might be by focussing on the International Standards, as these provide the mechanism for laying down benchmarks, it has been demonstrated that these are not necessarily up to date and it appears to take many years for advances in research to be adopted. In contrast, industry guidance can be more responsive to changes in technology and the emergence of new data. Therefore, an effort might be made to ensure that industry guidance reflects the most recent developments in corrosion research.

4. Typically, the operator guidelines do not conform with the latest developments (most noticeably for CSCC under evaporative films where temperature limits as low as 70°C and 80°C have been identified for super austenitic, duplex and super duplex steels). Moreover, those that do conform with International Standards (such as Norsok M-001:2004 [2]) may be non-conservative, particularly where an old version of the Standard is used. Again, this is because International Standards do not necessarily reflect the latest developments in corrosion research.

5. Throughout this work it has become apparent that most of the operators do not indicate in their guidance the source of the data on which the basis of their temperature limits have been derived. This constitutes a lack of transparency and makes it difficult to assess independently the validity of the values. This is also true for Norsok Standard M-001, although other International Standards, such as BS EN ISO 15156 and NACE MR0175, MR0176 do provide bibliographies that are better, to varying degrees, in explicitly referencing their source data.

6. The evidence indicates that current advances in corrosion science are being considered and some of the guidelines do now refer specifically to recent work where concentrated chlorides might lead to CSCC at temperatures lower than those currently set in International Standards. Another example, where current advances are being included is the recommendation that ‘ageing’ processes are carried out on components to develop a robust passive film prior to immersion in chlorinated seawaters. [2]

7. In certain operator guidelines and International Standards, the bottom-line for the specification of stainless steel alloys is that where doubt exists then technical expertise must be consulted or specific testing carried out on the alloy in question under the proposed service conditions.

8. There are many fewer data in the operator guidelines regarding temperature limits for localised corrosion (pitting and crevice corrosion), but where stated, they are in slightly better agreement than is the case for CSCC.

9. There is the added complication in that reported temperatures and temperature limits usually reflect the temperature of the environment rather than the all important temperature of the alloy surface.

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The operator and standards ‘critical’ temperature data for CSCC, localised (crevice and pitting corrosion) and sour service conditions have been incorporated into a summary table, overleaf, for four main stainless steel types namely: austenitic, super austenitic, duplex and super duplex. The table also presents the ranges of critical temperatures that have been obtained, to date, from the literature incorporated in the database spreadsheet. It is anticipated that the table will be revised as more data becomes incorporated in the spreadsheet.

Although not explicitly addressed in this work, it should be stated that the selection of stainless steels for offshore use should be on the basis of a suitable risk assessment to ensure that risks are kept as low as is reasonably practicable. The data that sit in the database provide a useful resource which can be consulted during this process.

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Chloride Environments Sour Operator Guideline or SCC Pitting and Crevice Corrosion Environment Standard External ° C Internal ° C °C °C A SA D SD A SA D SD A SA D SD A SA D SD Operator A - - - - - - - - - - - - - - - - Operator B 60 - 100 110 - - - - - - - - - - - 5 5 5 5 6 7 Operator C 60 80 - 100 90 - 110 80 - 100 90 - 110 - - - - - - - - 60 60,121-171 - 1 1 Operator D 50 - 70 /112 70 /112 - - - - - - - - - 150 150 6 7 Operator E 60 - 100 120 - - - - - 30NC/15C 30NC 60 60,121-171 - - Operator F 50 120 70 70 - 120 - - - 30 NC - 30NC/20C - - 2 2 2 2 6 7 Operator G 30 80 70 80 90 120 110 120 - - - 60 60,121-171 - - 4 3 4 3 Operator H 50 - 80 110 - - - - - - - 60-120 , 60-200 200 200 60 - 200 200

70 – 80 Literature Limits9 20 - 30 >50 - 200 70 – 80 ref 13 ref 13 ref 35 <-2.5 - 35 0 – 20 or 55 5 - 40 10 - 60 8

Norsok M - 001 60,70,85 120 100 110 85 20 120 150 150 150 6 7 BS EN ISO 15156-3 60 60, 121-171 232 232 BS EN ISO 21456 100 - 120 80 - 100 90 – 100 60 - 120 20

A = Austenitic Resumé of Temperature Limits for Offshore Operators and Standards SA = Super austenitic D = Duplex SD = Super duplex

C = Crevices present NC = No crevices present

1. 70°C for new plant, 112° for old 2. Hydrocarbon process fluids and produced water (these may constitute sour environment although separate data are given for sour service) 3. Vessels (varies depending on concentration of environmental species) 4. Pipework 5. The lower limit to be used in highly saline atmospheres 6. 60°C where pH2S is ≤ 100kPa.. Where chloride is < 50mg/l then there is no limit. 7. Temperature varies depending on concentration of environmental species 8. Temperature varies depending on concentration of environmental species, test method and severity of crevices (service data) 9. LITERATURE LIMITS are based on those 100 references whose data have been incorporated into the database to date. In excess of 100 further references remain to be examined and incorporated.

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1 INTRODUCTION

The Health and Safety Laboratory (HSL) has been commissioned by the Health and Safety Executive (HSE) Offshore Safety Division (OSD) to identify the safe operational temperature limits that are available to offshore oil and gas operators for the types of stainless steel alloys currently utilised in topside facilities. The driving force behind the work has been concern regarding the variability in the critical temperature values defined in the guidelines used by various operators. The work has been carried out in two parts. The first part has involved the generation of an information resource from literature in the public domain. The second part has consisted of a review of the operators’ temperature limits with those from the literature and Standards (such as Norsok M-001 [2] and BS EN 21457 [4] on materials’ selection, and BS EN ISO 15156 for sour service [3]) and identifying areas of greatest disparity. Together, it is hoped that these will be used to form the basis of operational guidance for inspectors.

In gathering data from the literature, the critical or maximum permissible-use temperature values for three corrosion mechanisms (i.e., pitting corrosion, crevice corrosion, and chloride stress corrosion cracking) have been obtained. These values are described respectively as:

• Critical Pitting Temperature (CPT), • Critical Crevice Temperature (CCT) and, • Critical Stress Corrosion Cracking Temperature (SCCT)

In addition, data that relate to more general thresholds have been included in the database. These have often arisen from observations of service failures and the operating temperatures under which the failure occurred, or laboratory-derived investigations examining corrosion behaviour under defined-temperature conditions.

The stainless steel alloy types examined have been:

• Austenitic (e.g., 316L, 304L) • Super austenitic (e.g., 254SMO or 654SMO) • Duplex (e.g., 2205 or 22Cr) • Super duplex (e.g., 25Cr)

While included in the scope of this work, few data have as yet been obtained for martensitic, ferritic and precipitation-hardening alloys.

In addition to evaluating data for chloride environments the author has also attempted to incorporate corrosion-related temperature limits for sour environments and, where available, to include the manufacturing condition and specific service of the alloys.

2 THE APPROACH TAKEN TO FORMAT THE DATA FROM THE LITERATURE

Owing to the broad remit of the work, and its ongoing nature, it has proved difficult to tabulate simply and concisely all the literature data into useable guidelines on temperature limits. In part, this is because of the large number of variables involved and the relationship between the variables being complex. An added difficulty is that data obtained in the laboratory can not necessarily provide temperature limits for actual service conditions. Presenting the data in a concise tabular format has not yet been practicable.

Work has been carried out previously [5] to provide guidelines on the use of stainless steels offshore and in that exercise the authors had primarily to leave the data in the raw state as to tabulate it would have created a construct of great complexity. The work reported here, however, has attempted some tabulation by entering the data into a database, presently in the form of a searchable EXCEL spreadsheet, thus enabling a swift isolation of data of interest. Notes at the end of each record provide the necessary fine detail from which an assessment of temperature limits and suitability for use might be obtained. Appendix 1 lists the field terms used in each record.

Not only does the database contain information regarding steel type and grade, and critical temperatures where available, it also contains information regarding the environment and applications under which, and for which, the data were derived. A large number of these data have come from papers published in scientific journals, but other sources in the public domain have been used and together these are listed in Appendix 2. These data then have provided the foundation for the review of the temperature-limit guidelines for stainless steel alloys currently in use by offshore operators and to assess where differences lie or how closely they reflect the latest limits available in the literature. This information is reported in Section 3 of this report, where recommendations regarding changes to the specified limits and areas for further work have been made. It is anticipated that this information will form part of HSE guidance regarding the appropriate use of stainless steel alloys offshore.

Resource limits have resulted in only 100 of the 210 references obtained from literature in the public domain, which contain critical temperature data, being transposed into the ongoing database thus far. Although this has generated over 500 individual records, it is clear that the database is far from complete and is less complete than is desirable. At this stage the database must be viewed as a working document and as funding permits more data will be entered. The order in which data has been incorporated has simply been on a basis of when it became available to the author. As such, this report should be regarded as the first of perhaps several instalments to be issued as the database is updated.

It is the author’s view that the database will provide a valuable repository of corrosion-related service failure data, in addition to laboratory derived data, from which records pertaining to particular alloys, environments, and usages will be extracted. The data will ultimately provide a valuable pan-sector resource that will contain the latest academic data regarding critical temperatures but, possibly of more value to HSE, recent and salient data surrounding individual in-service corrosion failures.

What has become evident from the data is that it is not possible to prescribe a single critical temperature for each of the three corrosion mechanisms (i.e., CSCC, pitting, and crevice corrosion) for a particular alloy grade in a given service environment. In large part, this is because the traditional ‘critical temperatures’ for a given alloy are generated from accelerated

laboratory tests using standard reagents and techniques and are often used as the means of ranking materials’ performance. In contrast, service conditions are uniquely diverse in all their variables. Standard laboratory tests, (e.g., ASTM G48, ASTM G150, etc.) have been shown to provide conservative temperatures [6-8, 49]. Under service conditions, however, these data have been shown to be inadequate in describing the operating envelope for the alloy in question. Defined limits, therefore, need to be specific to the environment and the material under consideration.

As advances in corrosion research reveal new mechanisms for corrosion under particular environmental conditions, it will be possible to include these new limiting conditions into materials’ selection and use guidance.

A driving force