Extremely corrosive environments, such as those often encountered in deep, hot, sour oil and gas wells, are usually characterized by the presence of hydrogen sulfide (H2S), carbon dioxide (CO2), chlorides, and other corrosive species coupled with high temperatures >400 deg F/204 deg C) and high pressures (up to 20,000 psi/138 MPa). Most low alloy and stainless steel materials are not suitable for such environments. Extremely corrosive service conditions dictate the use of a corrosion-resistant alloy (CRA) in areas which are exposed to the hostile environment. However, it is often cost-prohibitive to make an entire component out of CRA material. An alternative strategy is to use a low alloy steel for the bulk of the component and clad critical surfaces with a corrosion-resistant material. Clad equipment can provide excellent corrosion resistance in hostile environments at a fraction of the cost of 100% CRA components. This paper will detail the problems posed by extremely corrosive environments and discuss how clad equipment provides a cost-effective solution.
The corrosion problems posed by oilfield environments can be grouped into four general categories;
galvanic corrosion, and
Uniform Corrosion - Uniform corrosion is sometimes referred to as "general" or "weight-loss" corrosion since the surface of the metal is uniformly attacked (when viewed on a macroscopic scale). Uniform corrosion can lead to equipment failure by gradually reducing wall thickness until the component is no longer able to bear the imposed loads. A common type of uniform corrosion encountered in the oilfield is "sweet" or CO2 corrosion in which the metal surface is attacked by a carbonic acid solution formed by the reaction of CO2 with water.
Localized Corrosion - In contrast to uniform corrosion, localized corrosion affects a relatively small area. However, localized corrosion can lead to leakage in critical sealing areas. Pitting is a form of extremely localized corrosion which occurs when a small region of the metal surface becomes anodic to the surrounding metal, starting an autocatalytic or self-sustaining corrosion process. In oilfield environments, chloride ions often lead to pitting by causing localized breakdowns in the protective surface films formed by susceptible alloys such as stainless steels. Crevice corrosion is a type of localized attack which is often observed under gaskets, bolt heads, surface deposits, or similar regions where fluid flow is restricted. The concentration of metallic ions, oxygen and other gases, and dissolved salts in these stagnant areas becomes different from that of the surroundings resulting in accelerated corrosion.
Galvanic Corrosion - Galvanic or "dissimilar metals" corrosion occurs when two dissimilar metals are placed in contact with each other in an aqueous environment or other electrolyte. In most cases, the less noble of the two metals will corrode at an accelerated rate, especially if the surface area of the less noble metal is smaller than that of the more noble metal. The potential for galvanic corrosion can be estimated by comparing the relative positions of the two metals in the galvanic series, a table which ranks metals and alloys according to their electrode potentials in a given environment. Caution should be exercised in coupling materials which are far apart in the galvanic series (e.g., carbon steels and nickel-base alloys) when both materials will be exposed to the environment.
Environmentally-Induced Cracking - Environmentally-induced cracking is a major concern in the oilfield since it can cause components to fail suddenly and catastrophically, even though the corrosion rate (based on mass loss) is very low. Two environmentally-induced cracking mechanisms are commonly observed in sour wells, sulfide stress cracking (SSC) and anodic stress corrosion cracking (SCC). Sulfide stress cracking occurs when a metal absorbs hydrogen from H2S exposure causing a loss in ductility. If the embrittled material is then subjected to sufficient stress, cracking will occur. Anodic stress corrosion cracking is caused by the joint action of stress and corrosion. SCC typically originates at a site where anodic corrosion has created a pit or other defect which acts as a stress raiser and leads to the initiation of a crack in the presence of sufficient stress.