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99589 EFFECT OF MATERIAL COMPOSITION, TEMPERATURE AND POTENTIALS ON CREVICE CORROSION OF LC MARTENSITIC 13% CR STEELS IN WELD SIMULATED AND COLD DEFORMED CONDITIONS

Product Number: 51300-99589-SG
ISBN: 99589 1999 CP
Author: Hans Hoffmeister
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During operation of oil- and gasfields the new developed weldable low carbon martensitic 13% Chromium steels may be subjected to local corrosion by chloride containing wet phases. Crevice corrosion at defective welds, coatings or flanges may contribute to unexpected failures and shutdowns at temperatures and potentials below that of local pitting corrosion. In addition to established standardized test methods, crevice corrosion susceptibility can be quantified by measured corrosion starting times and corrosion currents. In the present investigation, both parameters are monitored by a ,,Remote Crevice Assembly” (RCA) providing quantitative consistence between local corrosion rates and measured net corrosion currents at a cathode/anode area ratio of 45000/1. Two materials with different Nickel and Molybdenum contents are tested at free corrosion conditions in CO2 purged formation water and nitrogen purged artificial seawater. Increasing potentials at test start are providing reduced crevice corrosion incubation times and increased corrosion currents, depending on fluid and type of steel. Weld simulating quenching from 1000°C of the anode provided stronger crevice corrosion only at the steel with higher Ni and Mo alloy content, which, otherwise was less crevice corrosion sensitive in the as delivered state as compared to the lower alloyed steel. Increasing temperatures of the cathode and the anode resulted in strong increases of corrosion currents. However, at temperatures above 40°C, pitting corrosion started at the cathode which resulted in reduction of crevice corrosion net currents. As a conclusion, the susceptibility to crevice corrosion of 13% Cr martensitic steels should be considered, also in possibly unintended pipeline operations and can be quickly established by short RCA tests. Kevwords: Martensitic 13% Cr steels, crevice corrosion, Remote Crevice Assembly
During operation of oil- and gasfields the new developed weldable low carbon martensitic 13% Chromium steels may be subjected to local corrosion by chloride containing wet phases. Crevice corrosion at defective welds, coatings or flanges may contribute to unexpected failures and shutdowns at temperatures and potentials below that of local pitting corrosion. In addition to established standardized test methods, crevice corrosion susceptibility can be quantified by measured corrosion starting times and corrosion currents. In the present investigation, both parameters are monitored by a ,,Remote Crevice Assembly” (RCA) providing quantitative consistence between local corrosion rates and measured net corrosion currents at a cathode/anode area ratio of 45000/1. Two materials with different Nickel and Molybdenum contents are tested at free corrosion conditions in CO2 purged formation water and nitrogen purged artificial seawater. Increasing potentials at test start are providing reduced crevice corrosion incubation times and increased corrosion currents, depending on fluid and type of steel. Weld simulating quenching from 1000°C of the anode provided stronger crevice corrosion only at the steel with higher Ni and Mo alloy content, which, otherwise was less crevice corrosion sensitive in the as delivered state as compared to the lower alloyed steel. Increasing temperatures of the cathode and the anode resulted in strong increases of corrosion currents. However, at temperatures above 40°C, pitting corrosion started at the cathode which resulted in reduction of crevice corrosion net currents. As a conclusion, the susceptibility to crevice corrosion of 13% Cr martensitic steels should be considered, also in possibly unintended pipeline operations and can be quickly established by short RCA tests. Kevwords: Martensitic 13% Cr steels, crevice corrosion, Remote Crevice Assembly
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