Qualification of Corrosion-Resistant Alloys for Sour Service

Abstract

ABSTRACT Prior to its use in sour production, a corrosion resistant alloy (CRA) must undergo a series of tests as part of an evaluation program to demonstrate suitability for service. Upon successful completion of this extensive evaluation program, the alloy is considered to be "qualified". The multi-step process of qualification and its guiding philosophy are presented in general as well a detailed discourse on testing CRA's with respect to resistance to environmentally assisted cracking. The paper concludes with examples of three CRA's that have been qualified; their limits of use in production environments are discussed. INTRODUCTION During the development of laboratory criteria and procedures to evaluate the sulfide stress cracking (SSC) resistance of low alloy steels, data on field service were abundant. Full scale commercial applications - successes and failures - were available to calibrate the Shell's Sc test for low alloy steels and to determine the threshold H2S partial pressure with respect to SSC [1]. Other materials, notably AISI 410 and 17-4 PH stainless steels, have shown resistance to SSC in some sour field applications prior to the development of laboratory screening procedures for low alloy steels. Although many have found that the SSC resistance of these and similar stainless steels to be lower than low alloy steels of comparable strength, many of these are included in NACE document MR0175 "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment" and are still found in (limited) use in sour service. Corrosion resistant alloys (CRA's) presented new problems in their qualification for service in the production of hydrocarbons from reservoirs containing H2S. First, most of the products were either newly developed composition or other (familiar) compositions that had been previously used only in the annealed form but were being introduced as heavily cold worked products to meet the required strength for downhole tubulars. Second, a data base of field performance did not exist, making it difficult to calibrate laboratory and prototype test techniques. Third, adequate corrosion resistance was required, which included absence of pitting and crevice corrosion, often under severe downhole conditions. And last, which is the focus of this paper, many CRA's were found to be susceptible to two distinct modes of environmentally assisted cracking: Stress corrosion cracking (SCC), a stress assisted, active path (anodic) process which is related to Cl SCC of stainless steels in hot brine solutions, but is accelerated by H2S, and Hydrogen embrittlement of high strength products, a cathodic process which is related to the classic SSC as cited above. This paper is a discussion of the laboratory procedures used to qualify an alloy for service in the production of hydrocarbons from reservoirs containing H2S. As this is a complicated process, the paper is limited to a presentation of the procedures and philosophy of evaluation of a CRA with respect to its resistance to environmental cracking (SCC and hydrogen embrittlement).