Propagation, corrosion

Because PTFE resins decompose slowly, they may be heated to a high temperature. The toxicity of the pyrolysis products warrants care where exposure of personnel is likely to occur (120). Above 230°C decomposition rates become measurable (0.0001% per hour). Small amounts of toxic perfiuoroisobutylene have been isolated at 400°C and above free fluorine has never been found. Above 690°C the decomposition products bum but do not support combustion if the heat is removed. Combustion products consist primarily of carbon dioxide, carbon tetrafluoride, and small quantities of toxic and corrosive hydrogen fluoride. The PTFE resins are nonflammable and do not propagate flame. 

Fig. 7. Crack velocity as a function of the applied stress intensity, Kj. Water and other corrosive species reduce the Kj required to propagate a crack at a given velocity. Increasing concentrations of reactant species shifts curve upward. Regions I, II, and III are discussed in text.

Depth of localized corrosion should be reported for the actual test period and not interpolated or extrapolated to an annual rate. The rate of initiation or propagation of pits is seldom uniform. The size, shape, and distribution or pits should oe noted. A distinction should be made between those occurring underneath the supporting devices (concentration cells) and those on the surfaces that were freely exposed to the test solution. An excellent discussion of pitting corrosion has been pubhshed [Corro.sion, 25t (January 1950)]. 

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

The weld was riddled with mildly undercut, gaping pits. Attack was confined to fused and heat-affected zones, with a pronounced lateral or circumferential propagation (as in Fig. 6.10). The resulting perforation at the external surface was quite small. Pits were filled with deposits, friable oxides, and other corrosion products. Black plugs embedded in material filling the gaping pit contained high concentrations of iron sulfide. Bulk deposits contained about 90% iron oxide. Carbonaceous material was not detected. 

SCC has been defined as failure by cracking under the combined action of corrosion and stress (Fig. 9.1). The stress and corrosion components interact S3mergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state. They may run in any direction but are always perpendicular to the principal stress. Longitudinal or transverse crack orientations in tubes are common (Figs. 9.2 and 9.3). Occasionally, both longitudinal and transverse cracks are present on the same tube (Fig. 9.4). Less frequently, SCC is a secondary result of another primary corrosion mode. In such cases, the cracking, rather than the primary corrosion, may be the actual cause of failure (Fig. 9.5). 

Several theories have been proposed to explain the corrosion-fatigue phenomena. One is that cyclic stressing causes repeated rupture of protective coatings. Corrosion-fatigue cracks propagate as the coating is successively reformed and ruptured along a plane. 

Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. 

Grain boundaries have different corrosion properties from the grain and may corrode preferentially, giving cracks that then propagate by stress corrosion or corrosion fatigue (Fig. 23.10). 

The toughness of wood is important in design for exactly the same reasons that that of steel is it determines whether a structure (a frame building, a pit prop, the mast of a yacht) will fail suddenly and unexpectedly by the propagation of a fast crack. In a steel structure the initial crack is that of a defective weld, or is formed by corrosion or fatigue in a wooden structure the initial defect may be a knot, or a saw cut, or cell damage caused by severe mishandling. 

Stress corrosion (see Section 2.3.4), in particular accelerated crack propagation in fatigue (see Section 2.3.5). 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

Because of their greater thickness, CAA oxides serve to protect the metal surface from corrosion better than thinner oxides but the important factor for bond durability is the stability of the outer oxide structure when water diffuses to the oxide-polymer interphase. Accordingly, it would be expected that the performance of CAA treated adherends would be similar, although no better, than that of PAA, or BSAA. The wedge test data shown in Fig. 20 and other work [29,77,97,98] support this and demonstrate that when these processes are done correctly the wedge test crack will be forced to propagate entirely within the adhesive. Similar arguments are likely with BSAA adherends, also. 

Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

Figure 4-441. Rate of stress—corrosion crack propagation as a function of crack depth during tensile loading. (From Ref. [183].)...

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