Cracking aqueous corrosion

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

Aluminum pistons in an engine that bums H2 will be exposed to not only H2 but also H2O at temperatures of 80 to 120°C. Aluminum alloys can be totally immune to H2 embrittlement and H2-induced crack growth if the natural AI2O3 oxide is intact. However, there are processes that can disrupt this film, and it is known that aluminum alloys will absorb H2 when exposed to H2O vapor at 70°C. There will also be periods when the engine is cool and condensed water will be present so that aqueous corrosion could occur, but this is not expected to be any different than with an engine with cast aluminum pistons that bums gasoline. 

Prior to the 1960s, stress corrosion cracking and corrosion fatigue were principally under the purview of corrosion chemists and metallurgists, and the primary emphasis was on the response of materials in aqueous environments (e.g., sea/salt water), particularly for SCC because of the relative ease of experimentation. Much of the attention was devoted to the understanding of electrochemical reactions that are associated with metal dissolution, crack nucleation, and time-to-failure under a... 

A typical rack employed for installation of specimens in pilot plants is shown in Fig. 6. Both corrosion coupons, 2 X 1 X 0.35 in. thick, and bend specimens intended to determine stress-corrosion cracking susceptibility, are included in the installation for aqueous corrosion testing. Specimens are separated by high density alumina spacers to eliminate electrochemical effects. During exposure, the racks are welded to existing components in the pilot plant equipment. 

There is a lack of fundamental understanding of the effect of elastic tensile stress or strain and plastic strain on dissolution by either thermodynamic or kinetic interpretations. The roles of stress and/or strain in aqueous corrosion reactions occurring at the atomistic level close to room temperature have been modeled for micrometer-scale descriptions of stress corrosion cracking (SCC) and hydrogen... 

These findings suggest that the failure mechanism is a combination of low-cycle corrosion fatigue and stress-induced corrosion. Extensive oxide formation relative to the depth of cracking is a key feature. The formation of oxide was associated with corrosion attack of the ferrite phase. The lamellar pearlite phase remained relatively intact and was contained within the oxide product. The oxide itself exhibited numerous cracks, allowing aqueous corrosion of fresh metal to occur at the oxide/metal interface. 

Brasses are susceptible to dezincification in aqueous solutions when they contain >15 wt% zinc. Stress corrosion cracking susceptibiUty is also significant above 15 wt % zinc. Over the years, other elements have been added to the Cu—Zn base alloys to improve corrosion resistance. For example, a small addition of arsenic or phosphoms helps prevent dezincification to make brasses more usefiil in tubing appHcations. 

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. 

The most important mechanism involved in the corrosion of metal is electrochemical dissolution. This is the basis of general metal loss, pitting corrosion, microbiologically induced corrosion and some aspects of stress corrosion cracking. Corrosion in aqueous systems and other circumstances where an electrolyte is present is generally electrochemical in nature. Other mechanisms operate in the absence of electrolyte, and some are discussed in Section 53.1.4. 

In practice, by far the most common case of stress corrosion is that occurring when austenitic stainless steels are simultaneously exposed to tensile stresses and hot, aqueous, aerated, chloride-containing environments. In this case the major variable is alloy composition and structure virtually all austenitic stainless steels are more or less susceptible to stress-corrosion cracking in these environments, while ferritic and ferritic/austenitic stainless steels are highly resistant or immune. 

Zehr, S. W., A Study of the Intergranular Cracking of U-7-5wt.%Nb-2-5wL Zr (Mulberry) Alloy in Aqueous Chloride Solutions , Corrosion, 28, 196 (1972)... 

Resistance to stress-corrosion cracking Commercially pure titanium is very resistant to stress-corrosion cracking in those aqueous environments that usually constitute a hazard for this form of failure, and with one or two exceptions, detailed below, the hazard only becomes significant when titanium is alloyed, for example, with aluminium. This latter aspect is discussed in Section 8.5 under titanium alloys. 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

The two principal forms of stress-corrosion failure are (a) hot salt cracking and (Z)) room-temperature cracking, the latter occurring in both aqueous and methanolic chloride environments, and in N2O4. In addition, environmental failures can occur in alloys in direct contact with some liquid and solid metals, and certain gases. 

Many titanium alloys are susceptible to stress-corrosion cracking in aqueous and methanolic chloride environments. 

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