Techniques for Measuring the End-Grain Corrosion Resistance of Austenitic Stainless Steels

Abstract

Stainless steels (SS) exhibit intergranular corrosion in hot, strong, nitric acid (HNO 3 ) solutions. The intergranular corrosion rate is infl uenced by the orientation of the microstructure, the highest rates typically being observed on plate, tube, and forging surfaces that are perpendicular to the hot-working direction. This is known as end-grain corrosion. In addition, pit-like tunnels can penetrate deeply beyond the broad front. In this study, a method is presented to deconvolute the corrosion rate obtained from the weight loss of coupons to investigate the differences in end-grain behavior for two austenitic SS, NAG (nitric acid grade) 18/10L (a Type 304L variant) and Uranus 65 (a Type 310L variant). In a corrosion simulant for dissolver liquor, the broad-front end-grain corrosion rate for the former steel was found to be ~2 mm/y, whereas for the latter it was ~1.1 mm/y. In addition, it is shown that measurement of the size and shape of drilled holes can be used to assess the effect of end grain on gross metal loss. For end-grain pits, neither of the above methods was found to be useful. Instead, sectioning or radiography is required. It was found that in the same test conditions, the maximum observed end-grain pitting rates were 7.0 mm/y for NAG 18/10L and 4.5 mm/y for Uranus 65. Uranus 65, however, showed a much smaller number of end-grain pits compared with NAG 18/10L. The cause of enhanced broad-front intergranular corrosion in the end-grain direction, as opposed to the pitting-like attack, is unknown to us.

Determination of the long-term intergranular corrosion rate of stainless steel in concentrated nitric acid

“…Ohno and co. obtained the same type of interface when they simulate the weight loss promoted by intergranular corrosion [17]. However from our point of view, this result is conclusive only when the material is subject to intergranular corrosion and not to end-grain corrosion [13].…”

“…In addition, during the immersion test, the groove depth H 0 is never higher than the grain size of the material. It means that the attack doesnot affect deeply the material, in contrast with the morphology of end-grain corrosion [13]. The groove angle is being constant and is roughly 25°for the two test solutions (Figs.…”

“…However, Dunned, Whillock and co. [10][11][12][13] emphasized recently that the weight loss is hitherto normalized with respect to the nominal surface area of the test specimen, instead of to the actual area of the metal undergoing corrosion. The latter increases with time and reaches a constant value after the onset of stable grain dropping.…”

Controlling End-Grain Corrosion of Austenitic Stainless Steels

“…Another form of corrosion to which particularly bar, wire, and tubular products of stainless steels are susceptible to attack in nitric acid environment, is end-grain corrosion. It takes place on the tubular and forged surfaces that are perpendicular to hotworking direction and occurs as localized pitting like attack that develop along the hot-working direction and finally the corrosion occurs as intergranular attack ( Ref 5,[7][8][9][10]. It has been identified (Ref 8) as a major form of corrosion in those components in which cross-sectional surfaces are exposed to the oxidizing process fluid.…”

“…Segregation of phosphorous, chromium and silicon along the flow lines during the fabrication stage is another mechanism for end-grain corrosion ( Ref 5,[9][10][11][12][13]. The end-grain corrosion of a material is related to the defect in manufacturing and processing stage such as a high-inclusion content in the material or use of an improper solution annealing heat treatment.…”