Corrosion-Resistant Alloy Cladding of Subsea Componets

Abstract

What is cladding? Cladding is defined as the process of protecting one metal by bonding a second metal to its surface.1 Why is cladding utilized in subsea components? Cladding supplies a combination of desired properties not found in any one metal. A base metal can be selected for cost or structural properties and another metal added for some special property such as corrosion protection. For example, a majority of subsea components are designed to meet a minimum yield strength of 75 ksi and are constructed from low alloy chrome moly steels such as UNS G41300. Although the mechanical properties of 4130 are excellent and it has been the bell weather base material in the oil and gas industry for the past 25 plus years, it is not known for its corrosion properties. On the other hand Alloy 625 (UNS N06625) is known for its excellent corrosion properties, but it only has a yield strength of 60 ksi minimum. If you combine both alloys, 4130 as the structural component and Alloy 625 cladding as the corrosion component, both desired characteristics are attained. The cladding of Subsea production components with Corrosion Resistant Alloys (CRA) is performed to protect the base material from corrosion, thus assuring the structural integrity of the components over the proposed life of the field. Considering the cost of recovery, loss of production and the replacement/refurbishment of subsea components, it is prudent to protect them from corrosion. What is the difference between cladding, inlay and overlay? There are no differences; all three terms are used interchangeably, which sometimes creates confusion. Historically the application of Corrosion Resistant Alloy Cladding has been a viable, cost effective and reliable method of corrosion protection that has been incorporated into oil and gas components over the past 50 years. Cladding was originally used to combat crevice corrosion and was applied in ring grooves and seat pockets. Up until approximately 15 years ago the predominant cladding material was 300 series austenitic stainless steel grades. API 6A2 explicitly details the qualification of cladding welding procedures, which include acceptable chemical composition and the distance from the base material where the chemistry should be sampled. The most common cladding material in the austenitic stainless steel grades is Type 316L. The "L" grades are used to provide extra corrosion resistance after welding. The letter "L" after a stainless steel type indicates low carbon. To avoid carbide precipitation in the material when heated in the critical range of 800° F to 1600° F carbon is kept to 0.03% or less. Coincidently PWHT of cladded components are conducted at the lower end of this temperature range. Carbon precipitates out of the cladding and combines with the chromium which gathers on the grain boundaries, thus denying the material of chromium in solution, which in turn promotes corrosion adjacent to the grain boundaries. By controlling the amount of carbon in the cladding filler material the migration of carbon is minimized.