Intergranular corrosion testing of austenitic stainless steels in nitric acidsolutions

摘要

In hot strong nitric acid solutions, stainless steels exhibit intergranular corrosion. Corrosion ratesrnare often measured from immersion testing of specimens manufactured from the relevant materialrn(e.g. plate or pipe). The corrosion rates, measured from weight loss, are found to increase withrntime prior to reaching steady state, which can take thousands of hours to achieve. The apparentrnincrease in corrosion rate as a function of time was found to be an artefact due to the surface arearnof the specimen's being used in the corrosion rate calculations, rather than that of the true arearnundergoing active corrosion I.e. the grain boundaries. The steady state corrosion rate coincidedrnwith the onset of stable grain dropping, where the use of the surface area of the specimen tornconvert the weight loss measurements to corrosion rates was found to be appropriate. This wasrnconfirmed by sectioning of the specimens and measuring the penetration depths. The rate ofrnpenetration was found to be independent of time and no induction period was observed. A methodrnwas developed to shorten considerably the testing time to reach the steady state corrosion rate byrnuse of a pre-treatment that induces grain dropping. The long-term corrosion rates from specimensrnwhich were pre-treated was similar to that achieved after prolonged testing of untreated (I.e.rnInitially ground) specimens.rnThe presence of cut surfaces is generally unavoidable in the simple immersion testing ofrnspecimens in test solutions. However, inaccuracy in the results may occur as the measuredrncorrosion rate is often influenced by the orientation of the microstructure, the highest ratesrntypically being observed on the cut surfaces. Two methods are presented which allowrndeconvolution of the corrosion rates from immersion testing of specimens containing cut surfaces,rnthus allowing reliable prediction of the long-term corrosion rate of plate surfaces.