Influence of hydrogen on corrosion and stress induced cracking of stainless steel

Abstract: Hydrogen is the smallest element in the periodical table. It has been shown in several studies that hydrogen has a large influence on the corrosion and cracking behaviour of stainless steels. Hydrogen is involved in several of the most common cathode reactions during corrosion and can also cause embrittlement in many stainless steels. Some aspects of the effect of hydrogen on corrosion and hydrogen-induced stress cracking, HISC, of stainless steels were studied in this work. These aspects relate to activation of test specimens for uniform corrosion testing, modification of a test cell for dewpoint corrosion testing and the mechanism of hydrogen-induced stress cracking.The results from uniform corrosion testing of superduplex stainless steels indicated that there is a large difference between passive and activated surfaces in hydrochloric acid and in lower concentrations of sulphuric acid. Hence, initial activation of the test specimen until hydrogen evolution can have a large influence on the results. This may provide another explanation for the differences in iso-corrosion curves for superduplex stainless steels that have previously been attributed to alloying with copper and/or tungsten. In concentrated sulphuric acid, potential oscillations were observed; these oscillations activated the specimen spontaneously. Due to these potential oscillations the influence of activation was negligible in this acid.An experimental set-up was developed for testing dewpoint corrosion of stainless steels in a condensate containing 1 % hydrochloric acid. There was an existing experimental set-up that had to be modified in order to avoid azeotroping of the water and hydrogen chloride system. A separate flask with hydro chloric acid was included in the experimental set-up. The final set-up provided reasonably good agreement with field exposures in contrary to much higher corrosion rates in the original set-up.Relaxation and low temperature creep experiments have been performed with several stainless steels in this work. The aim was to understand how creep and relaxation relates to material properties and the relative ranking between the tested materials. For low temperature creep with a load generating stresses below the yield strength, as well relaxation at stress levels above and below the yield strength, the same ranking with respect to changes in mechanical properties of the steel grades was found. For low temperature creep with a load level above the yield strength, the same ranking was not obtained. This effect can most probably be explained by annihilation and generation of dislocations. During low temperature creep above the yield strength, dislocations were generated. In addition, low temperature creep experiments were performed forone superduplex stainless steel in two different product forms with differentaustenite spacing in the microstructure. The superduplex material experienced low temperature creep at a lower load level for the material with large austenite spacing compared to the one with smaller austenite spacing. Also this differenceis influenced by dislocations. In a material with small austenite spacing the dislocations have more obstacles that they can be locked up against.Studies of the fracture surfaces of hydrogen induced stress cracking, HISC, tested duplex stainless steels showed that HISC is a hydrogen-enhanced localised plasticity, HELP, mechanism. Here a mechanism that takes into account the inhomogeneous deformation of duplex stainless steels was proposed. This mechanism involves an interaction between hydrogen diffusion and plastic straining. Due to the different mechanical properties of the phases in a superduplex stainless steel, plastic straining due to low temperature creep can occur in the softer ferrite phase. A comparison between low temperature creep data showed that for the coarser grained material, HISC occurs at the load levelwhen creep starts. However, in the sample with small austenite spacing, HISC did not occur at this load level. Microhardness measurements indicated that the hydrogen level in the ferrite was not high enough to initiate cracking in the coarser material. The proposed mechanism shows that occurrence of HISC is an interaction between local plasticity and hydrogen diffusion.