Corrosion conditions for

The formation of a layer of metal oxide on the surface of this steel provides better corrosion resistance in oxidizing environments than under reducing conditions. Common steels 304, 304L, 347, 316 and 316L are used for equipment exposed to aqueous solutions of acids and other low-temperature corrosive conditions. For high-temperature regimes involving... 

Material Properties. Materials possess various mechanical and chemical properties, and, therefore, it is possible to select materials appropriate for severe corrosion conditions. For example, if the equipment is under cyclic loading, a material with high fatigue strength is desired. Similarly, it is desirable to have corrosion-resistant materials for the corrosive environments. There are several sources for obtaining information on materials properties. Some are listed in Table 4-173. 

Substitute carbon steel shafts by stainless steel or by lead-bonded steel in corrosive conditions for longer life. 

These may be of wood, metal, chemical stoneware, acidproof brick, glass, plastic, plastic- or glass-lined metal, or other material depending upon the corrosion conditions. For ease of construction and strength they are usually circular in cross section. 

Martensitic steeis. The corrosion resistance of martensitic stainless steels is moderate (i.e., better than carbon steels and low-alloy steels but inferior to that of austenitic steels). They are typically used imder mild corrosion conditions for handling water, steam, gas, and oil. The 17% Cr steels resist scaling up to 800°C and have low susceptibility to corrosion by sulfur compounds at high temperatures. 

This guide covers one-coat shop painting systems for steel that will not be exposed to corrosive conditions for long periods. They are also suitable for steel encased in concrete in those oases where bonding of steel to concrete is not required. They can be used under fireproofing. These systems are suitable for use on parts or structures exposed in Environmental Zones 0 (encased in concrete or masonry, normally dry) and lA (interior, normally dry). The paints covered by this guide are primers, and if a color other than the standard color is required, the color must be specified. 

ISO document ISO/TC 156 NWIP Corrosion of Metals and AUoys-Test Method for Thermal Cycling Exposure Testing under High Temperature Corrosion Condition for Metalic Materials this document is based on the results of the COTEST project)... 

ISO/TC 156 NP 21608 Corrosion of metals and alloys— Test method for isothermal exposure testing under high temperature corrosion conditions for metallic materials. International Organisation for Standardisation, Geneva, 2006. 

The techmque was first employed, in real-world conditions, for monitoring external corrosion in the large diameter steel tubing used for oil well casings. In the late fifties, T.R. Schmidt at Shell Developments, pioneered the technique in those demanding applications, although the technique itself was invented, by W.R. MacLean, (Ref. 1) somewhat earlier. T.R Schmidt has written a history (Ref. 2) of much of the early work in the technology, which contains many references, others which may be of interest are held on the NTIAC database (Ref 3). 

Laser-based profilometry systems have also been adapted for unique applications in nuclear power generating plants. Applications where quantitative information with regard to surface condition for mechanisms such as surface pitting and flow-assisted corrosion are candidates for this NDT method. 

Technetium is a silvery-gray metal that tarnishes slowly in moist air. The common oxidation states of technetium are +7, +5, and +4. Under oxidizing conditions technetium (Vll) will exist as the pertechnetate ion, TcOr-. The chemistry of technetium is said to be similar to that of rhenium. Technetium dissolves in nitric acid, aqua regia, and cone, sulfuric acid, but is not soluble in hydrochloric acid of any strength. The element is a remarkable corrosion inhibitor for steel. The metal is an excellent superconductor at IIK and below. 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

Titanium is resistant to nitric acid from 65 to 90 wt % and ddute acid below 10 wt %. It is subject to stress—corrosion cracking for concentrations above 90 wt % and, because of the potential for a pyrophoric reaction, is not used in red filming acid service. Tantalum exhibits good corrosion resistance to nitric acid over a wide range of concentrations and temperatures. It is expensive and typically not used in conditions where other materials provide acceptable service. Tantalum is most commonly used in appHcations where the nitric acid is close to or above its normal boiling point. 

The reaction is facilitated by elevated temperature necessitating pressure-capable, glass-lined reactors and exotic metallurgy for fittings to withstand the severely corrosive conditions. PAG product having 10—12% Al as AI2O2 can be produced. 

Types 321 and 347 have additions of titanium and niobium, respectively, and are used in welding appHcations and high temperature service under corrosive conditions. Type 304L may be used as an alternative for Types 321 and 347 in welding (qv) and stress-reHeving appHcations below 426°C. 

Aluminum geodesic dome roof tanks are becoming popular. These are often the economic choice. They offer superior corrosion resistance for a wide range of conditions, and are clear span stmctures not requiring internal supports. They can also be built to any required diameter. However, domes caimot handle more than a few inches of water column internal or external pressure. 

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

Corrosion of boiler tubes appears to be initiated in some cases with the formation of a white layer of general composition (Na,K)2Al(S0 2-Conditions for initiation of the deposit are favored by coals having high alkaU and sulfur contents. The white layer bonds to the tubes and permits growth of ash deposits that insulate the layer and permit further corrosion. 

Other Equipment Cost ModiEers. Temperature, pressure, or corrosive conditions can act as modifiers of the base cost by requiring thicker vessel walls, more expensive alloys, special seals, more expensive fabrication, and special testing procedures. Separate materials and process severity factors for temperature, pressure,and material,, multiply the base (mild steel) cost as ... 

The Electrolytic Corrosion Test. Also developed for use on nickel—chromium and copper—nickel—chromium decorative automobile parts is the electrolytic corrosion (EC) test (44). Plated specimens or parts are made anodic in a corrosive electrolyte under controlled conditions for 2 min, and then tested for penetration to the substrate. 

Fiber from PPS resins has been made in two forms. Monofilament is used in paper machine drier felts to replace polyester, which is attacked by the hot, corrosive conditions of papermaking. Staple fibers are made into filter bags for flue treatment, and are considered a growth area. 

Clad Tube Sheets Usually tube sheets and other exchanger parts are of a solid metal. Clad or bimetallic tube sheets are usecito reduce costs or because no single metal is satisfactory for the corrosive conditions. The alloy material (e.g., stainless steel, Monel) is generally bonded or clad to a carbon steel backing material. In fixed-tube-sheet construction a copper-alloy-clad tube sheet can be welded to a steel shell, while most copper-alloy tube sheets cannot be welded to steel in a manner acceptable to ASME Code authorities. 

Certain corrosive conditions require that special consideration be given to complete drainage when the unit is talcen out of service. Particular consideration is required for the upper surfaces of tube sheets in vertical heat exchangers, for sagging tubes, and for shell-side baffles in horizontal units. 

Two types of mechanical-draft towers are in use today the forced-draft and the induced-draft. In the forced-draft tower the fan is mounted at the base, and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the ran and drive outside the tower, where it is convenient for inspection, maintenance, and repairs. Since the equipment is out of the hot, humid top area of the tower, the fan is not subjected to corrosive conditions. However, because of the low exit-air velocity, the forced-draft tower is subjected to excessive recirculation of the humid... 

In the absence of factual corrosion information for a particular set of fluid conditions, a reasonably good selection would be possible from data based on the resistance of materials to a very simifar environment. These data, however, should be used with some reservations. Good practice calls for applying such data for preliminary screening. Materials selected thereby would reqmre further study in the fluid system under consideration. 

Heat Flux Tests Removable tube test heat exchangers find an ideal use in the field for monitoring heat flux (corrosion) conditions, NACE TM0286-94 (similar to laboratory test. Fig. 28-4, page 28-12). 

Unsuitable. Not available in form required or not suitable for fabrication requirements or not suitable for corrosion conditions. 

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