High-temperature hydrogen attack

Mention was made earlier in this chapter of hydrogen attack being a potential life-limiting degradation mechanism. As experience indicates that this can manifest itself in reactors, pipework and heat exchangers, it is expedient to deal with the available assessment procedures in a common section. 

Component susceptibility to hydrogen attack is a function of material chemical composition, miCTOStructural response to fabrication heat treatments. 

Assessment procedures are available for evaluating the effect of quantified levels of hydrogen attack on vessel integrity. These are mainly based on a remaining sound ligament approach and do not rigorously take account of 

This chapter has illustrated that plant assessment technology tools can provide results, quantified in most cases in probability of failure versus time terms, that give the Inspection, Maintenance and Operations functions a basis on which to optimise the cost benefit of their decisions and actions, both prior to and during shutdown of the plant and which can be deployed within an integrated framework for through-life plant management. 

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Both industry experience and research work indicate that postweld heat treatment (PWHT) of chromium-molybdenum steels in hydrogen service improves resistance to high temperature hydrogen attack. The PWHT stabilizes alloy carbides. This reduces the amount of carbon available to combine with hydrogen, thus improving high temperature hydrogen attack resistance. 

The amount of hydrogen partial pressure reduction depends upon the materials and the relative thickness of the cladding/ weld overlay and the base metal—the thicker the stainless barrier is relative to the base metal the better.32 Archakov and Grebeshkova33 mathematically considered how stainless steel corrosion barrier layers increase resistance of carbon and low alloy steels to high temperature hydrogen attack. 

There have been a few instances of high temperature hydrogen attack of base metal which was clad or overlayed with austenitic stainless steel. All of the reported instances involved C-0.5Mo steel base metal. In one case,34 high tem-... 

APPENDIX A—HIGH TEMPERATURE HYDROGEN ATTACK OF 0.5Mo STEELS... 

No instances have been reported of high temperature hydrogen attack of Mn-0.5Mo steel below the Figure A-l 0.5Mo curve. The information and use of this material at elevated temperatures and hydrogen partial pressures are limited. 

Table A-1—Operating Conditions For C-0.5Mo Steels That Experienced High Temperature Hydrogen Attack...

APPENDIX B—HIGH TEMPERATURE HYDROGEN ATTACK OF 1.25 Cr-0.5Mo STEEL... 

The purpose of this appendix is to provide a brief summary of the information and experience regarding three case histories with high temperature hydrogen attack of 1,25Cr-0.5Mo steel. 

The three recent experiences with high temperature hydrogen attack are listed in Table B-l, and the operating condi tions are plotted in Figure B-1. 

Table B-1—Experience with High Temperature Hydrogen Attack of 1.25Cr-0.5Mo Steel at Operating Conditions Below the Figure 1 Curve...

APPENDIX C—HIGH TEMPERATURE HYDROGEN ATTACK OF 2.25CM Mo STEEL... 

High Temperature Hydrogen Attack In A Liquid Hydrocarbon Phase. 8... 

C-l—Experience with High Temperature Hydrogen Attack of 2.25Cr-lMo... 

D-1 —Summary of Inspection Methods for High Temperature Hydrogen Attack. 27... 

At elevated temperatures, molecular hydrogen dissociates into the atomic form, which can readily enter and diffuse through the steel. Under these conditions, the diffusion of hydrogen in steel is more rapid. As discussed in Section 4, Forms of High Temperature Hydrogen Attack, hydrogen may react with the carbon in the steel to cause either surface decarburization or... 

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