Technical Note:Microstructure Effects in the Stress Corrosion Cracking of Alloy 28 Under Sour Conditions

Abstract

Alloy 28 (UNS N08028), an important candidate material for sour gas well service, was investigated for sensitivity to stress corrosion cracking under a sour environment simulating the down-hole service conditions. Three different heat samples were tested, and their microstructure was observed with an optical microscope and scanning electron microscopy (SEM); microchemistry was analyzed using energy-dispersive spectroscopy (EDS). Specimens with precipitates from heat A failed in stress corrosion cracking (SSC) testing, and their specimens without precipitates from heats B and C did not. The precipitated phase was rich in carbon, chromium, and molybdenum, and with high hardness. Some microcracks were observed around the platelet precipitates. The microstructure had a remarkable effect on the SCC of Alloy 28 under sour conditions, so the optimum microstructure was the key to Alloy 28 use in sour service conditions.

The Effect of Precipitates on Corrosion Resistance of Alloy 28 Tubing in Sour Environments

“…Corrosion evaluation included SCC test and IGC test using the specimens shown Fig.1 [16] . The slow strain rate test (SSRT) method was selected for SSC test because it would provide crack orientation in same plane as occurs in field failures, and can obtain some information on SCC more easily than other test method.…”

“…High-temperature, high-pressure environments containing with sulfide (H 2 S) and high chlorine ion increasingly challenge the ability to service these wells using carbon steel or low alloy steel casing and tubing. The microstructure and microchemistry affect the intergranular corrosion (IGC) and stress corrosion cracking of nickel base alloys [9][10][11][12][13][14][15][16][17][18] . For decades nickel base alloys have been successfully used in highly corrosive wells, consistent with their for the superior corrosion resistance and excellent mechanical properties [1][2][3][4] .…”