The role of passive film growth kinetics and properties in stress corrosion and crevice corrosion susceptibility

Abstract

The entry of hydrogen into a metal's lattice is known to lead to the phenomenon called hydrogen embrittlement for some alloy systems.It is not known, however, why some alloys suffer such hydrogen damage and others do not.It is not even clear which alloys do suffer such damage.For example, it is controversial as to whether the stress corrosion of austenitic stainless steel is due to hydrogen entry into the alloy's lattice or to the mechanisms not involving hydrogen. Moreover, if hydrogen damage is known to occur, its mechanism if, indeed, there is a single mechanism, is poorly understood and controversial (1).In order to provide further insights into how hydrogen entry into a lattice produces conditions that lead to sub-critical crack growth, it would be valuable to examine on an atomic scale if possible, the rearrangements new phase formations and other disturbances that hydrogen produces when it enters a metal lattice.The possibility for making such examinations exists through the use of field ion microscopy (FIM).Recently, this laboratory completed a study on the effects of hydrogen entry into the lattice of titanium using FIM (2).The results obtained by FIM for the beginning steps of hydrogen entry agreed with earlier eletron microscopy studies with regard to the crystal lographic relationships found for titarium hydride formation. In the Ti studies the hydrogen was introduced as a gas by using it as an imaging gas.The stress required for hydrogen entry was provided by the high fields present when using FIM. Hancock and Johnson (3) have shown that in microscopic systems sub-critical crack growth can be produced in low pressure hydrogenThe work described here seeks to extend the earlier Ti study by applying the FIM to an examination of the effect of gaseous hydrogen on a number of alloys and uranium, a metal known to suffer extensive hydrogen damage.In the group of alloys studied are 1080 steel, 304 stainless steel and Fe-24% Cr alloy. EQUIPMENTThe field ion microscope was equipped with a 75 mm diam microchannel plate image intensifies and a specimen-quick-change chamber for rapid specimen insertion and withdrawal.Routinely, images from two specimens could be obtained within a half an hour's time.This was an important advantage in the following studies involving the hydrogen interaction with several types of specimens, pure metals and alloys, subjected to a variety of environmental and preimaging treatments promoting hydrogen/metal interactions.Furthermore, the images obtained were generally unstable, thus requiring image intensification in order to achieve the necessarily fractional second, photographic exposure times. The specimen temperatures below room temperature were controlled by adjusting the flow of cold gaseous helium from a liquid helium dewar over specimenmount-feed-through pin.During field ion microscopy reagent grade imaging 1 gases were continuously flowed through the imaging chamber. shows a field ion micrograph of 1080 steel imaged with a helium-neon gas mixture.Boundaries (arrows) appear...