Corrosion mechanism layer

Scale is a layer or layers of minerals deposited onto a heat transfer surface, which reduces the heat transfer coefficient (U value, stated in Btu/hr/sq ft/°F). In everyday parlance, scale also refers to thick layers of corrosion product built up onto a metal surface (often present in association with deposits) that may occur at high temperature as a result of a variety of boiler surface corrosion mechanisms. 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

The relative significance of the individual deposition mechanisms for corrosion of materials may vary in different areas depending e.g. on the distance from the emission source, and also for different materials depending on differences in corrosion mechanisms and nature of protective layers of corrosion products. 

The formation of a corrosion product layer on a membrane surface can have significant impacts on the overall membrane performance (and mechanical life), but the infiuence relies heavily on the structure/formation of the scale. For example, the surface scale may only partially cover the membrane surface, such as a porous scale or a scale exhibiting cracks resulting from relieving surface stresses (Fig. 10.11). The partially covered membrane surface may still exhibit enough catalytic sites to promote the adsorption and dissociation of hydrogen, but may yield a smaller amount of atomic hydrogen due to the reduced active metal surface area. 

The starting point for such classification is the point of interference with the above sketched corrosion mechanism either in a phenomenological or in a mechanistic way, A simple system for classification, which will be discussed in more detail later, is based on whether the inhibitor interferes with the anodic or cathodic reaction. Thus inhibitors are classified as anodic or cathodic inhibitors. However, this distinction was shown to be too simplistic and a more complex classification was worked out by H. Fischer (JJ on the basis of where, instead of how, in the complex interphase of a metal-electrolyte system the inhibitor interferes with the corrosion reactions. The metal-electrolyte interphase can be visualized as consisting of (a) the interface per se, and (b) an electrolyte layer interposed between the Interface and the bulk of the electrolyte. On this basis Fisher distinguished as shown in Table 1, between "Interface Inhibition" and "Electrolyte Layer Inhibition."... 

In hydrochloric acid for instance at room temperature the corrosion rate at 95% inhibition is still several hundred mpy s. Therefore.under steady state conditions there is a flux of iron ions across the interphase which must be accommodated in the inhibition mechanism. This could be done more easily by assuming a corrosion product layer of discrete thickness composed of a complex formed from metal ions and inhibitor. Details of this model will be discussed below. 

Differences in detected Volta potentials between pristine and corroded Al-Mg alloy surfaces could be related to the factors influencing thickness and conductivity of the corrosion product layers [219]. Corrosion layers developed in the presence of ion-containing solutions yielded lower Volta potentials and showed higher conductivity. Cathodic delamination of poly aniline-based organic coatings on iron have been studied with SKP [220]. The role of dioxygen reduction and of the poly aniline fraction in the coating were included in a proposed corrosion mechanism. 

Mechanical stability of corrosion product layer Where the corrosion deposit has insufficient mechanical stability to maintain the thickness required to achieve dynamic equilibrium between the corrosion and dissolution rates, particles of the deposit will break off on an intermittent basis producing peaks of contamination. This type of breakdown would only be expected after prolonged ageing (>70 days for brasses) of the test coupon. Determining the difference between the total metal leached into the test solution and its filtered metal content could give an indication of the presence of this problem. 

Many factors contribute to the corrosion resistance of a modern painted steel or aluminum product including the nature of organic coating, the metal substrate and/or metal coating, and the conversion layer. It is impossible to speak of the corrosion resistance afforded by the conversion layer without reference to the total product. In this section, we will briefly describe some phenomena unique to the conversion layer, which do contribute to the corrosion resistance. A detailed description of corrosion mechanisms under paint is given in Chapter 5.4. 

The diffusion constant D is determined by the concrete quality. At the carbonation front there is a sharp drop in alkalinity from pH 11-13 down to less than pH 8. At that level the passive layer, which we saw in Chapter 2 was created by the alkalinity, is no longer sustained so corrosion proceeds by the general corrosion mechanism as described in the Chapter 2. 

Tang et al. studied these effects using DFT (with a complementaiy in situ STM study) [95]. They found similar trends for the Pt nanoparticles studied compared to the bulk in relation to metal/hydroxide/oxide stability but found a very strong dependence on particle size. For the smallest particle size studied (radius of 0.25 nm), at low pH, there is a direct dissolution pathway and hydroxides and oxides are not predicted to form prior to dissolution. Consequently, it can be surmised that under fuel cell conditions, there is a crossover in particle size below which the particles do not develop a passivating oxide layer and instead are subject to direct dissolution, thus decreasing their aheady compromised stability. This study again only looks at surface occupation of OH and O without surface/particle reconstruction but provides very useful information for the stability of Pt nanoparticles and the conditions that lead to oxide formation at the nanoscale. In addition, it shows how DFT can not only give information on phase stability but also point to corrosion mechanisms under different conditions. 

It is obvious that the scaling resistance of the heat-resisting steels will be detrimentally influenced by any other corrosion mechanism which may be destroying the oxide layer, e.g., by chemical reactions with other metal oxides, chlorine, or chlorides. Thus in general, the heat resistance cannot be characterized by a single test method or measuring parameter but will depend on the speciflc environmental conditions. 

If the critical shear stress that acts on the layer of corrosion products present at a surface is exceeded, mechanical film damage leads to a strong increase in the rate of corrosion. According to this view, stainless steel and titanium are not susceptible to flow accelerated corrosion because the thin passive oxide films formed on these metals are more resistant to shear stresses than the corrosion product layers found on copper and its alloys [16]. 

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