Environmental factors, stress corrosion

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

Buoyancy in some form is employed in nearly all categories of underwater and surface systems to support them above the ocean bottom or to minimize their submerged weight. The buoyant material can assume many different structural forms utilizing a wide variety of densities. The choice of materials is severely restricted by operational requirements, since different environmental conditions exist. For example, lighter, buoyant liquids can be more volatile than heavier liquids. This factor can have a deleterious effect on a steel structure by accelerating stress corrosion or increasing permeability in reinforced plastics. 

When sensitive metals are exposed to certain environmental factors or tensile stress, corrosion can occur. Exposure to only a few parts per million of a corrosive agent can initiate stress corrosion in some metals. High concentrations are often not necessary. Also, temperature and pH can be influencing factors. 

The environment also plays a role in some environments brittle crack failure is strongly promoted. For example, detergents such as synthetic soaps can decrease the time to brittle failure of PE by a factor between 10 and 50 (see Figure 7.21). This phenomenon is known as stress corrosion or environmental stress cracking (ESC) (see further 8.5). 

Additionally, specific environmental conditions can induce localized corrosion such as temperature, conductivity of the corrosive fluid, or thickness of the liquid corrosive film in contact with the metal. In some cases, both metallurgical and geometric factors will influence behavior, such as in stress-corrosion cracking. Preferential weldment corrosion of carbon steels has been investigated since the 1950s, commencing with the problems on icebreakers, but the problem continues today in different applications. (Bond)5... 

Wear is the process of physical loss of material. In sliding contacts this can arise from a number of processes in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. Wear occurs because of local mechanical failure of highly stressed interfacial zones and the mode of failure is influenced by environmental factors. 

K/-V relations observed in this study are shown in Figure 3, where results obtained under two different water vapor pressures were shown. It can be seen that the reproducibility of testing results of Kumamoto andesite under the same environmental condition is very high from this figure. In addition, it can be seen that the crack growth behaviors are facilitated under high water vapor pressure. This is in harmony with the theory of stress corrosion cracking in rocks. The obtained results are summarized in Table 1, where K/ means K, at V = 10 [m/s], and v means v at Ki = 1.6 [MN/m ]. Ki and v were determined in order to compare the stress intensity factor... 

K scc depends on metallurgical factors (it usually decreases as the strength of the steel increases, even though it also depends on the microstructure of the material, e. g. it is lower in quenched and tempered steel than in cold-worked steels) and on environmental factors (for instance, in alkaline environments and in the absence of chlorides, Kfscc so high that normal mechanical failure takes place before stress corrosion cracks can develop). 

Environmental stress cracking is similar, but not identical to, stress corrosion cracking of metals. Corrosion involves chemical reactions that produce corrosion products, whereas, in ESC, a liquid is absorbed by the polymer, promoting crazing and crack formation. Corrosion reactions are rare in polymers. ESC can typically cause a factor-of-ten reduction in strength. The two conditions for it to occur are that... 

WUde, B. F. (1981) Stress corrosion cracking of ASIM A517 steel in liquid ammonia Environmental factors. Corrosion, 37, No. 3. 

Durability of adhesive materials is affected by environmental factors. The specific environments of concern are (1) extreme high temperatures, (2) extreme low temperatures, (3) extreme high humidity, (4) salt water, (5) fire, (6) corrosive gases or liquids, and (7) external stresses. A detailed discussion of the first five factors is given. Future research needs about these factors are also suggested. 

Stress-corrosion cracking is caused by the interaction of metallurgical, mechanical, and environmental factors, with the result that there is a multiplicity of possible S.C.C. control measures that can be implemented. In addition, the complexity of S.C.C. has led to a very large number of hypotheses, models, theories, and controversies on the mechanisms by which S.C.C. occurs. These matters will be summarized in this section. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

In this chapter, the applicable environmental factors include temperature, vapor phase and condensed moisture, corrosive inorganic gases, organic vapors, particles, electric fields, and air velocity. While vibration, mechanical stress, thermal shock, and solar radiation can occasionally contribute to materials degradation in indoor environments, these subjects are covered in other chapters and MIL-STD-810E. 

Pelensky, M. A. and Gallaccio, A., "Stress Corrosion of Magnesium Alloys—Environmental Factors," in Stress Corrosion Testing, ASTM STP 425, ASTM International, West Consho-hocken, PA, 1967, pp. 107-115. 

Uniform surface corrosion, i.e. corrosion at a nearly uniform corrosion rate over the entire surface, is usually less problematic from an operational point of view. This factor can be taken into account in the structural element design in the form of an anticorrosive additive and can be controlled in many structural elements by means of regular wall thickness measurements, e.g. by ultrasonic means. Much more difficult problems result from local corrosion types such as pitting corrosion and stress corrosion cracking (SCC). The corrosion types are difficult to control and can rapidly lead to failure of structural elements after only a low level of mass loss. Damage from such corrosion types are rarely predictable and not only cause considerable losses in economic terms but also entail risks to safety and environmental protection. This applies in particular to system elements that must function under pressure. 

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