Pipeline corrosion resistance alloy

It is usefiil to consider the case of an installation of a subsea gathering system for a natural gas production field. The pipeline design for a new gas production facility consisted of 20 cm diameter subsea gathering lines (flow lines) emptying into a 19 km, 50 cm diameter subsea transmission gas pipeline. The pipeline was to bring wet gas from an offshore producing area to a dehydration facility on shore. The internal corrosion was estimated to be 300-400 mpy. The corrosion mitigation options considered were (i) carbon steel treated with a corrosion inhibitor (ii) internally coated carbon steel with a supplemental corrosion inhibitor (iii) 22% Cr duplex stainless steel (iv) 625 corrosion-resistant alloy (CRA). The chance for success was estimated from known field histories of each technique, as well as the analysis of the corrosivity of the system and the level of sophistication required for successful implementation (Table 4.42). 

Despite these qualifications copper and its alloys are used extensively and successfully in much chemical equipment. Uses include condensers and evaporators, pipelines, pumps, fans, vacuum pans, fractionating columns, etc. Tin-bronzes, aluminium-bronzes and silicon-bronzes are used in some circumstances because they present better corrosion resistance than copper or brasses. 

High-alloy pipeline steels (e.g. austenitic-ferritic or duplex) have been used where the product stream demands materials with better corrosion resistance than carbon steel. In practice the external corrosion resistance of these materials cannot be guaranteed, so cathodic protection is employed to protect areas which may be subject to corrosion. 

Economic losses are divided into (1) direct losses and (2) indirect losses. Direct losses include the costs of replacing corroded structures and machinery or their components, such as condenser tubes, mufflers, pipelines, and metal roofing, including necessary labor. Other examples are (a) repainting structures where prevention of rusting is the prime objective and (b) the capital costs plus maintenance of cathodic protection systems for underground pipelines. Sizable direct losses are illustrated by the necessity to replace several million domestic hot-water tanks each year because of failure by corrosion and the need for replacement of millions of corroded automobile mufflers. Direct losses include the extra cost of using corrosion-resistant metals and alloys instead of carbon... 

Fuel assemblies installed in their channels consist of two subassemblies connected in series. The container type fuel rods are filled with pellets of low enrichment uranium dioxide with the addition of a burnable absorber (erbium). Fuel claddings are made of zirconium alloy (Zr 1% Nb), and channel tubes inside the core are fabricated from another zirconium alloy (Zr 2.5% Nb). Corrosion resistant steel is employed for the inlet and outlet pipelines of the channels outside the core. 

The fuel channel tube is installed so that its zirconium part (Zr 2.5%Nb alloy) is located inside the core, with the part made of corrosion resistant steel outside. The acceptance criterion addressed in this section is only applicable to the zirconium part. Steel portions of the fuel channel are treated as circulation circuit pipelines according to the respective acceptance criteria (Section 4.6). 

An acceptable life for undergound pipelines can sometimes be realized through the use of corrosion-resistant piping. Copper, aluminum, and stainless steel piping are sometimes used for this purpose. All three alloys can have greater corrosion resistance than carbon steel or cast iron. However, they are not immune to corrosion and are often more susceptible to localized corrosion such as pitting, crevice corrosion, and SCC than carbon steel or cast iron. Further, corrosion protection in the form of coatings and cathodic protection are frequently used. 

Corrosion damage to several components made of low-alloy steels and cast iron in the waste water treatment plant of a petroleum refinery were also caused by the action of sulphate-reducing bacteria. To prevent further damage, the pipelines were either replaced by those made of glass-fiber-reinforced polyester resin or they were lined with sulphate-resistant concrete [12]. 

The choice of the appropriate material is decisive for resistance against microbially influenced corrosion. This means that before the choice of material can be made, what kind of impacts is has to resist needs to be considered. Microbial influencing factors must also be considered. Accordingly, in the presence of volatile sulfur compounds, e.g., in sewage pipelines, it is recommended not to use materials like unprotected concrete which may be destroyed by the end product of the microbial degradation process (in this case, sulfuric acid formed by Thiobacilli). Another example would be the choice of a stainless steel or of an alloy that cannot be attacked under the conditions of a biofllm and the complex metabolic processes occurring underneath it. If, for instance, a material has to be chosen for static reasons, this material has to be protected by a coating or a liner made of an inert material. All these examples are based on the consideration that all attack factors have been identified by a complete inventory. 

Soils will pit steels, which obviously affects buried pipelines. In one study of 10 carbon and low-alloy carbon steels containing Cr, Ni, Cu, and Mo and exposed to a variety of soils for 13 years, the conclusion was that factors such as soil pH, resistivity and degree of aeration have more influence on the severity of corrosion than the alloy content of the steel. In any case, protective coatings and cathodic protection are the best means of reducing corrosion in buried pipelines. 

Machinery and equipment. Aluminum is used in processing equipment in the petroleum industry such as aluminum tops for steel storage tanks and aluminum pipelines for carrying petroleum products. It is also used in the rubber industry because it resists all corrosion that occurs in rubber processing and is nonadhesive. Aluminum alloys are widely used in the manufacture of explosives because of their nonpyrophoric characteristics. Aluminum is used in textile machinery and equipment, paper and printing industries, coal mine machinery, portable irrigation pipe and tools, jigs, fixtures and patterns, and many instruments. 

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