Carbon steel weld corrosion

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

Estimate the thickness required for the component parts of the vessel shown in the diagram. The vessel is to operate at a pressure of 14 bar (absolute) and temperature of 300°C. The material of construction will be plain carbon steel. Welds will be fully radiographed. A corrosion allowance of 2 mm should be used. 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

NACE Standard RP0472, Methods and Controls to Prevent In-Service Cracking of Carbon Steel Welds in (P-1) Corrosive Petroleum Refining Environments/ NACE, Houston, TX, latest revision. 

C. The material of construction will be plain carbon steel. Welds will be fully radiographed. A corrosion allowance of 2 mm should be used. 

Galvanostatic None Carbon steels Preferential corrosion of weld metal and of HAZ... 

Some alkylation processes use concentrated HF instead of H2SO4 as the catalyst. In general, HF is less corrosive than HCl because it passivates most metals by the formation of protective fluoride films. If these films are destroyed by dilute acid, severe corrosion occurs. Therefore, as long as feedstocks are dry, carbon steel - with various corrosion allowances - can be used for the vessels, piping, and valve bodies of hydrofluoric acid alkylation units. AU carbon steel welds that will contact HF, should be post-weld heat treated. 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

The wall thickness selected varies with the service and material. Carbon steel coils are often made from schedule 80 or heavier pipe to allow for corrosion. When stainless-steel or other high-aUoy coils are not subject to corrosion or excessive pressure, they mav be of schedule 5 or 10 pipe to keep costs at a minimum, altliougli high-quahty welding is required for these thin walls to assure trouble-free seiwice. 

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. 

Amine systems are extremely corrosive due to the acid-gas concentrations and the high temperatures. It is important that all carbon steel expo.sed to the amine be stress-relieved after the completion of welding on the particular piece. A system fabricated from stress-relieved carbon steel for DEA solutions, as recommended, will not suffer excessive corrosion. For MEA systems, corrosion-resistant metals (304 SS) should be used in the following areas ... 

GENERAL MATERIAL MAXIMUM DESIGN PRESSURE and TEMPERATURE LIMITS UMITED BY CORROSION ALLOWANCE CONSTRUCTION Carbon Steel 275 PSIG at -20/I00°F 100 PSIG at 750°F 150 Flanges See Table, This Spec. ly" and Smaller—Socket Welded 2" and Larger—Flanged and Butt-Welded... 

Cihal, V., Explanation of Intercrystalline Corrosion of Welded, Stabilised Cr-Ni Stainless Steels , Bergakademie, 15, 23 (1963) C.A., 58, 13523d Fleitman, A. H., Romano, A. J. and Kiermut, C. J., Corrosion of Carbon Steel by High Temperature Liquid Mercury , J. Electrochem. Soc., 110, 964 (1963)... 

Stainless steel welds should provide reliable joints that are internally smooth and corrosion free. Low carbon stainless steel, compatible... 

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