Retardation of hydrogen embrittlement of 17-4ph stainless steels by nonmetallic surface layers
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G.T. MURRAY, J. P. BOUFFARD, and D. BRIGGS
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In a previous article ~ it was demonstrated that thin (-500 nm thickness) sputtered layers of TiO2 onto 15-5 PH* stainless steels effectively prevented hydrogen em-
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*PH is a trademark of ARMCO Steel Corporation. >-
brittlement (HE) as measured by ductility loss on electrolytically charged specimens. The purpose of the present communication is to describe the results of the beneficial effects of other sputter-coated materials, namely silicon oxide, aluminum oxide, and silicon nitride, plus thermally grown oxide films. The purpose of the latter study was to attempt to develop a more practical coating approach, and in particular to determine if the oxide film could be grown during the normal aging heat treatment. The literature on the effects of oxides on hydrogen entry and/or HE has been previously reviewed.I References that should be added include the work of Swansiger and coworkers 2'3 and Stone, 4 whose data indicate that a low hydrogen surface adsorption coefficient plays a leading role in preventing hydrogen entry. Caskey s and Piggott and Siarkowski, 6 on the other hand, showed very low diffusivities ( - 10-12 to 10-17 cm 2 s-l) of hydrogen through oxides. It may well be that both phenomena contribute to the ability of oxide films to retard hydrogen entry. No data on the effect of carbide surface layers could be found in the literature. Tensile specimens (0.64 mm diameter by 25.4 mm gage length) of 17-4 PH stainless steel were prepared as described in previous publications. 1,7 They were aged four hours at 566 ~ (1050 ~ to as H1050 condition) in atmospheres of either air, argon, or oxygen. One specimen was aged for 20 hours in oxygen. The air aged specimens were reactively sputter coated to thicknesses of 50 and 200 nm of SiO2, Si3N4, and A1203. Cathodic charging was done by immersing the specimens in a 3.5 pet NaCI solution containing approximately 30 mg/1 of As203 as a poison. A current density of 90 A/m 2 from a constant current source was employed for a period of 16 hours. Two specimens were gaseously charged with hydrogen at 475 K and a hydrogen pressure of 13.8 MPa for 24 hours. Tensile tests at a cross-head speed of 1.27 mm/min were conducted within 10 minutes of completion of the charging operation. The percent ductility loss, defined as: pet R.A. without hydrogen--pet R.A. with hydrogen pct R.A. without hydrogen for A1203, SiO2, and Si3N4 sputter coated specimens as a function of film thickness is depicted in Figure 1. The average pet R.A. obtained on control specimens, i.e., without
G.T. MURRAY is Professor of Metallurgical Engineering, California Polytechnic State University, San Luis Obispo, CA 93407. J.P. BOUFFARD is Engineer II, Northrop Aircraft Corp., Hawthorne, CA. D. BRIGGS is Associate Engineer, IBM, San Jose, CA. Manuscript submitted January 28, 1986. 162--VOLUME 18A, JANUARY 1987
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