Corrosion-resistant alloys, design

Design of Chemical Plant in Wiggin Nickel Alloys, and Wiggin Corrosion Resisting Alloys, Henry Wiggin Co. Ltd. 

In order to determine the corrosion state of an active-passive system, the position of the corrosion potential relative to pp must be determined. According to Fig. 4, if Econ is below Ew, the material will undergo uniform dissolution under film-free conditions. If EC0II is above Epp but below Et, the material will be passive and will dissolve at its passive current density, which is often on the order of 0.01 mpy. Corrosion-resistant alloys are designed to operate under such conditions. For situations in which Ec0II is above Et, the material will dissolve transpas-sively, i.e., uniformly. 

Corrosion, the degradation of a material s properties or mass over time because of environmental effects, is a costly reality that effects every industry. A study issued by the Federal Highway Administration (FHWA) in 2002 conservatively estimates the annual direct cost of corrosion in all U.S. industry sectors at US 276 billion. Costs associated with corrosion include cathodic/anodic protection coatings inhibitors corrosion-resistant alloys and materials and maintenance, repair, and depreciation of equipment. Indirect costs, such as lost productivity, environmental or product contamination, planning and design, and lost opportunities, can easily outpace direct costs by factors of two or more. 

The more expensive, corrosion-resistant alloys are frequently used as a cladding on carbon steel. If a thick plate is needed for structural strength, as for pressure vessels, the use of clad materials can substantially reduce the cost. The design requirements for pressure vessels with cladding or applied internal linings are given in ASME BPV Code Sec. VIII Div. I Part UCL. 

In the case of tanker trucks and railcar-mounted tanks, linings and corrosion allowances for internal corrosion are used. In the case of transportation of corrosive materials corrosion-resistant alloys are used. In extreme cases, rubber bladder tanks have been used on flat-bed trailers or railcars. External corrosion is controlled with coatings and designs that minimize crevices. One possible solution to minimize crevice corrosion is by placing a horizontal tank with a circular cross-section on legs, thus avoiding direct contact with other surfaces. 

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). 

In fact, there are many ways that can be thought of as routine strategies that, nowadays, industries use globally to manage corrosion. Cathodic/anodic protection, application of coatings, use of the so-called corrosion-resistant alloys (CRAs), as well as improvements in design are among the most important ones. 

The original conductor materials were stainless steel or platinum. These materials were later replaced with more corrosion-resistant alloys with improved fatigue resistance such as MP35N (SPS Technologies Cleveland OH), an alloy of nickel, chromium, cobalt, and molybdenum. In order to further reduce the resistance to current flow, specialized conductors were designed, including DBS (drawn brazed strand) and DFT (drawn filled tube)... 

Design and modeling of new corrosion-resistant alloy chemistries and structures. 

Craig, B. D., Collins, J. C., Patrick, R. L., and Gilbert, T., Testing and Evaluation of Corrosion-Resistant Alloys, Materials Selection and Design, December 1991, pp. 51-55. 

The oxychlorination reactor is a vertical cylindrical shell made of carbon steel with a support grid/air sparger system and internal cooling coils. Internal or external cyclones are used to minimize catalyst carryover. The reactor internal parts are made from corrosion-resistant alloy. The reactor has many design features depicted in Fig. 1. 

A good level of understanding of the surface reactions involved in the formation of passive films (passivation/repassivation) is necessary for designing new, highly corrosion-resistant alloys, and this is achieved by active, continuous, and worldwide research efforts in this area. 

Some of the nickel-based alloys were designed to withstand high temperature and dry or gaseous corrosion whereas others are mainly designed to resist low-temperature (aqueous) corrosion. Nickel-based alloys used for low temperature aqueous or condensed systems are generally known as corrosion-resistant alloys (CRA). Only the corrosion performance of commercially available CRA will be addressed in this chapter. 

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