Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic Stainless Steels in Chloride and Caustic Environments

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Maintenance Best Practice Guide: GBHE_MBPG_1614

Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic Stainless Steels in Chloride and Caustic Environments

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Maintenance Best Practice Guide: Avoiding Stress Corrosion Cracking of Carbon Low Alloy and Austenitic Stainless Steels in Chloride and Caustic Environments CONTENTS 0 PURPOSE 1 SCOPE 2 KEY DEFINITIONS

3 RELEVANT DOCUMENTATION 4 SPECIFIC LEGAL REQUIREMENTS 5 FACTORS PROMOTING STRESS CORROSION CRACKING (SCC)

IN CHLORIDE AND CAUSTIC ENVIRONMENTS 6 AVOIDING STRESS CORROSION CRACKING (SCC)

IN CHLORIDE AND CAUSTIC ENVIRONMENTS

6.1 CONTROL OF STRESS

6.2 CONTROL OF ENVIRONMENT 7 SUMMARY



APPENDICES A GUIDANCE ON HYDROTEST WATER QUALITY FOR AUSTENITIC

STAINLESS STEEL EQUIPMENT TO AVOID CHLORIDE SCC B GUIDANCE ON THE PREVENTION OF CHLORIDE SCC FOR

AUSTENITIC STAINLESS STEEL HEAT EXCHANGERS ON COOLING WATER DUTIES

C SUMMARY FOR GENERAL GUIDANCE ON SCC OF CARBON STEELS

AND AUSTENITIC STAINLESS STEELS IN CHLORIDE AND CAUSTIC ENVIRONMENTS

FIGURES 1 SCC OF AUSTENITIC STAINLESS STEELS AS A FUNCTION OF

CHLORIDE CONCENTRATION AND TEMPERATURE 2 CAUSTIC SODA SERVICE FOR AUSTENITIC STAINLESS STEELS 3 GBHE CAUSTIC SODA SERVICE CHART FOR CARBON/LOW ALLOY

STEELS 4 SCC AT TUBE/TUBEPLATE JOINTS DUE TO CONCENTRATION OF

CHLORIDE OR CAUSTIC (HYDROXIDE).



0 PURPOSE

The purpose of this Maintenance Best Practice Guide is to assist engineers to avoid stress corrosion cracking when designing or specifying equipment, to aid in diagnostic work and to guide inspection personnel dealing with equipment which may hold fluids containing chloride or caustic (hydroxides).

1 SCOPE

This Maintenance Best Practice Guide is concerned with the performance of carbon, low alloy steels, and austenitic stainless steels, in fluids containing chloride or caustic (hydroxides). Those factors which are known to promote stress corrosion cracking are outlined, and service charts defining environmental boundaries for stress corrosion cracking in chloride and caustic containing fluids are presented. General guidance on the avoidance of stress corrosion cracking is provided.

The avoidance of 'external' stress corrosion cracking of austenitic stainless steels beneath thermal insulation is covered in a separate Maintenance Best Practice Guide.

2 KEY DEFINITIONS Stress Corrosion Cracking Cracking Cracking produced by the (SCC) combined action of corrosion

and tensile stresses. 3 RELEVANT DOCUMENTATION GBHE-MBPG-1814 Materials of Construction Review GBHE-MBPG-1914 Unfired Fusion Welded Pressure Vessels GBHE-MBPG-2014 Light Duty Items in Stainless Steel GBHE-MBPG-0714 The Pressure Testing of In-Service Pressurized

Equipment.



4 SPECIFIC LEGAL REQUIREMENTS

Account shall be taken of relevant local legislation. 5 FACTORS PROMOTING STRESS CORROSION CRACKING (SCC) IN

CHLORIDE AND CAUSTIC ENVIRONMENTS

The common grades of carbon and low alloy steels are vulnerable to stress corrosion cracking (SCC) in fluids containing sodium or potassium hydroxide (caustics). They are not vulnerable to SCC in chloride containing fluids, but can suffer from general corrosion. High strength grades of steel (>950MPa UTS) can suffer SCC due to hydrogen produced by corrosion reactions on their surfaces, including those induced by chloride containing fluids. However, this topic is beyond the scope of this Best Practice Guide,

All of the common grades of 18%Cr, 8-10%Ni austenitic stainless steel, including types 304, 304L, 316, 316L, 321 and 347, are vulnerable to SCC in both chloride and caustic containing fluids. Other grades of stainless steel, including the more highly alloyed austenitic grades (e.g. those grades containing >25Ni such as types 904L and 825) and 'duplex' grades (i.e. those containing 5-7%Ni such as types 2205 and 2507) are also vulnerable to SCC in chloride and caustic containing fluids, but are beyond the scope of this Best Practice Guide. Guides to safe operating conditions in chloride and caustic containing fluids are shown in Figures 1 to 3. Although this data can be used for general guidance, it is important to be alert to the effects of local concentration and heating. Various factors can concentrate otherwise benign levels of corrodent to concentrations which promote SCC including:

(a) boiling and evaporative concentration due to locally high heat fluxes/skin

temperatures; (b) intermittent wetting and drying of surfaces associated with liquid injection

into hot pipelines, level variation in storage vessels, etc.; (c) the presence of crevices and surface deposits leading to diffusional

concentration.



Residual fabrication stresses commonly trigger SCC at welds and at cold worked areas, including cold-formed bends, pressed components, expansions, etc. Highly stressed equipment such as bellows are also particularly vulnerable. Heat exchangers of all types are potentially vulnerable to SCC. In the case of tubular exchangers, problems are more likely if the corrodent is on the shellside, with heat transfer into the tube/tubeplate crevice. Favored cracking sites are in, and adjacent to tube/tubesheet welds, and at the overlap zones of the expansion stages. Vertical or inclined bundles can be particularly vulnerable to such problems if the top tubesheet is inadequately vented, and a vapor space develops, as shown schematically in Figure 4. In such cases, even high quality demineralized waters with trace levels of chloride or caustic can concentrate and promote cracking in time periods as short as a few days. In the case of chloride induced SCC of austenitic stainless steels, the qualities of waters used for pressure testing and cooling are significant issues. Some general guidance is provided in Appendices A and B. 6 AVOIDING STRESS CORROSION CRACKING IN CHLORIDE AND

CAUSTIC ENVIRONMENTS 6.1 Control of Stress In the case of carbon and low alloy steels, tensile stresses around yield are required to promote SCC, which as a result is commonly associated with residual rather than operating loads. Cracking can then be prevented by appropriate stress relief procedures, of which thermal stress relief is the more widely practiced and effective. Equipment design codes contain procedures for thermal stress relief, but do not give information on when it is required to prevent SCC.



In the case of austenitic stainless steels, stress corrosion cracking can be initiated by relatively low stresses. Thermal stress relief in austenitic stainless steels requires much high temperatures than carbon steels, it is therefore considered to be a less practicable procedure than for carbon steels. As a result, thermal stress relief is rarely used to control SCC in austenitic stainless steels. Dissimilar welds between carbon or low alloy and austenitic materials cannot be thermally stress relieved effectively, and their use should be avoided in circumstances where they are vulnerable to SCC. The use of mechanical stress relief procedures such as shot peening can be beneficial in specific circumstances. However, such techniques are specialized and require careful control; a materials engineer should be consulted about their use whenever possible. 6.2 Control of Environment Figures 1 to 3 provide guidance on safe operating windows to avoid SCC of equipment, but care is needed to ensure that local heating/concentration effects are anticipated successfully at the design stage and avoided. In certain circumstances, corrosion inhibitors can be used to reduce the propensity of an environment to promote SCC, e.g. the use of phosphates in waters. However, there are many traps for the unwary, and some factors which have resulted in SCC in practice which were not anticipated at the design stage are: (a) Injection of water with traces of free chloride or caustic to desuperheating

steam, resulting in concentration due to wetting/drying of downstream surfaces;

(b) Failure to appreciate that some sources of caustic have high chloride

levels; (c) Hydro testing equipment with poor quality water; (d) Allowing excessive fouling of tubular heat exchanger shellside surfaces,

resulting in local heating/concentration; (e) Gagging back on water supply to heat exchangers to control process

temperatures resulting in local heating/concentration; (f) Using high temperature tubeside process cleaning procedures resulting in

dry out and therefore concentration, of water in the shellside.



7 SUMMARY The main factors outlined in this Good Practice Guide are summarized in Appendix C. Stress corrosion cracking is a complex topic, and a materials engineer should be Consulted whenever possible.

Effect of Cu-content on crack growth rate. The effect of initial stress

on time to failure of maraging steel in 3.5% NaCI solution.



APPENDIX A GUIDANCE ON HYDROTEST WATER QUALITY FOR AUSTENITIC STAINLESS STEEL EQUIPMENT TO AVOID CHLORIDE SCC

Control of the quality of hydro test water is necessary for the prevention of stress corrosion cracking by chlorides present in the test water, which remain in the equipment after the hydraulic test. The greatest susceptibility is at high chloride concentrations and service temperatures more than 60oC. (a) Do not use salt or contaminated water for testing, e.g. borehole, river,

canal, lake, estuarine, or sea water. (b) When the process operating temperature is less than 60oC then use of

potable quality test water is acceptable. (c) When the operating temperature is above 600C but the metal is flushed by

process fluids or condensing steam at start up, then the use of potable quality test water is acceptable, provided the equipment is completely self-draining.

(d) When the operating conditions are above 60oC and the metal is not

flushed by process fluids or condensing steam on start up, then, provided the equipment is completely self-draining, test water of potable quality can be used, followed by flushing with water containing less than 1ppm chloride.

(e) When the operating temperature is above 60oC, and the vessel is not

flushed by process fluids or condensing steam and is NOT completely self-draining, test water with less than 1ppm chloride should be used. Dry out carefully by swabbing, and/or blowing with warm air (less than 60oC).

(f) Reference (c) and (d) above, enclosed spaces such as shell sides of heat

exchangers or vessel or pipe jackets which operate above 600C should always be tested with water containing less than 1ppm chloride. Experience dictates that residual chloride cannot be removed successfully by flushing.

Note: Less than 1ppm chloride = demineralized water, or pure condensate.



APPENDIX B GUIDANCE ON THE PREVENTION OF CHLORIDE SCC FOR AUSTENITIC STAINLESS STEEL HEAT EXCHANGERS ON COOLING WATER DUTIES

The '300' series of 18%Cr, 8-10%Ni austenitic stainless steels are used commonly for heat exchanger construction. Their weakness is their susceptibility to stress corrosion cracking in chloride-containing media, and, materials engineers are often asked, 'What is the critical level of chloride ions which stainless steel will tolerate?’ This is not a question to which there is a simple answer. A much easier question to answer is, ‘what is the critical temperature below which chloride stress corrosion cracking will not occur?' There is no unique answer to this question either, but most materials engineers would agree that below 700C stress corrosion cracking is unlikely, and below 600C it is extremely rare. For this reason, prevention of stress corrosion cracking in heat exchangers in the Company cooling systems has been based upon control of skin temperature rather than chloride level per se. In broad terms, the chloride contents of the Company cooling systems in the US are generally < 200 ppm, and rarely, if ever, exceed 600 ppm. In Europe, somewhat higher levels have to be tolerated, but even so, chloride contents rarely exceed 800 ppm. Where flow rates are high and crevices/deposits are absent, i.e. when cooling water flow is through the tubes, US experience indicates that skin temperatures up to 100OC can be tolerated in such fluids. However, stress corrosion cracking commonly initiates from pitting or crevice/deposit corrosion, and is thus favored by low flow conditions and occluded areas associated with joints, scales, foulants, etc. Thus for water-in-shell bundles or plate exchangers, the only reliable defense against stress corrosion cracking is to restrict skin temperatures to a maximum of 600C. In practice most cases of stress corrosion cracking in austenitic stainless steel heat exchangers are associated with operational practices resulting in higher skin temperatures and/or higher chloride contents than anticipated at the design stage. Thus, excessive fouling of shellside tube surfaces, restricting water flow to adjust process temperatures, high temperature tubeside cleaning resulting in shellside dry out etc, etc, can lead to failure by stress corrosion cracking in systems which would have otherwise proved benign. If you have any doubts about the integrity of your own system, contact a materials engineer.



APPENDIX C SUMMARY FOR GENERAL GUIDANCE ON SCC OF CARBON STEELS AND AUSTENITIC STAINLESS STEELS IN CHLORIDE AND CAUSTIC ENVIRONMENTS

CHLORIDE ENVIRONMENTS (CL¯ )



CAUSTIC ENVIRONMENTS (OH¯ )

Notes: 1 The term austenitic stainless steels as used in this note includes grades

304, 304L, 316, 316L 321 and 347. 2 The term carbon steels includes carbon-manganese and low alloy steels.



FIGURE 1 SCC OF AUSTENITIC STAINLESS STEELS AS A FUNCTION

OF CHLORIDE CONCENTRATION AND TEMPERATURE



FIGURE 2 CAUSTIC SODA (SODIUM HYDROXIDE) SERVICE CHART FOR

AUSTENITIC STAINLESS STEELS



FIGURE 3 GBHE CAUSTIC SODA (SODIUM HYDROXIDE) SERVICE

CHART FOR CARBON/LOW ALLOY STEELS



FIGURE 4 SCC AT TUBE/TUBEPLATE JOINTS DUE TO

CONCENTRATION OF CHLORIDE OR CAUSTIC (HYDROXIDE)

