Selection guidelines for corrosion resistant alloys in the oil and gas industryThe selection of Corrosion Resistant Alloys, CRAs, for producing and transporting corrosive oil and gas can be a complex procedure and if improperly carried out can lead to mistakes in application and misunderstanding about the performance of a CRA in a specific service environment. There is a variety of ways individuals and companies select CRAs for anticipated well and flowline conditions. Companies with large research facilities typically initiate a test program that involves simulating the particular part of the field environment under study (i.e., flowlines versus downhole). Then a group of alloys, based on information available, is selected that represents a possible range of alternatives. Rather than test all alloys all the time, it is more cost-effective and less time- consuming to test only a few CRAs that are likely candidates. This approach can easily require 1 to 3 years to accomplish at considerable expense.
Another selection procedure is to review the literature for corrosion data that generally applies to the anticipated field conditions. This can result in elimination of those CRAs that are not good candidates and, thus, narrow the number of candidate alloys for testing. The selected CRAs are then tested under very specific conditions to fill gaps in literature data and/or field experience. Care must be taken when using this approach because, for example, the corrosion resistance of many CRAs at one temperature is not necessarily indicative of their corrosion resistance at other temperatures. Likewise, changes in critical environmental components such as elemental sulphur can have a profound impact on the resistance to stress corrosion cracking, SCC, another important factor in alloy selection.
The quickest and least expensive alloy selection method is simply to review the literature, and existing or similar field data, and make the selection. This method can be quite unsatisfactory since certain critical factors or conditions will not be known and must be assumed. A greater chance for error exists in this selection approach, introducing a potential for failure of the CRA or use of a more expensive alloy than is required. It is advisable, if this method is used, to consult with someone who has a working knowledge of CRAs and their applications. Finally, a CRA selection method that is not recom- mended but is often used is to select a CRA that is readily available or most economical, without regard to its corrosion resistance in the intended environment. Misapplication of CRAs is becoming more common for this reason and has resulted in corrosion and cracking problems of the inappropriately selected alloys........
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