Corrosion corrodent forms

Zinc is susceptible to attack from oiQ gen concentration cells. Shielded areas or areas depleted in oxygen concentration tend to corrode, forming voluminous, white, friable corrosion products. Once the zinc layer is breached, the underlying steel becomes susceptible to attack and is severely wasted locally (Figs. 5.12 and 5.13). 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Localized corrosion, another form of metal waste, generally occurs on small, confined areas of metallic bodies. Localized corrosion processes expand mostly by penetrating deep into the bulk of the metal, forming holes and cracks that may endanger the integrity of corroded objects (see Textbox 43). 

Although AI protects itself against corrosion by forming a natural oxide, the protection is not complete. In the presence of moisture and electrolytes, AI alloys, particularly the high-copper alloys, corrode much more rapidly than pure Al. 

CHEMICAL PROPERTIES noncombustible liquid hydrolyzes in water to form hydrochloric acid and phosphoric acid highly corrosive corrodes most common construction materials reacts with chemically active metals such as sodium, potassium and aluminum reacts vigorously with strong nitric acid strong oxidizer FP (NA) LFLZUFL(NA) AT (NA) HC (NA) HF (-319.7 kJ/mol liquid at 25°C) H 7.1 kJ/mol at 161K). 

The vast majority of corrosion inhibitors in neutral environment as well as a number of acid corrosion inhibitors form protective 3D films on the metal surface ( interphase inhibition [4]). These films may consist of adsorbate multilayers, ox-ide/hydroxides, salts, or reaction products formed by interaction of the inhibitor with solution species on or near the corroding metal surface (e.g. dissolved metal ions). The type, structure, and thickness of the inhibiting films are strongly influenced by the environmental conditions. The interphase films act as a physical barrier that blocks or retards transport processes and the kinetics of the corrosion reactions at the metal surface. The inhibitive properties could, in some cases, be correlated with the chemical stability of the corresponding insoluble complexes as well as with the solubihty, adsorbabOity, and hydrophobicity of the inhibitor molecules [35]. Often, other ions from the electrolyte, such as... 

Corrosion Initiation Defects in coatings are always preferential sites for corrosion initiation. Apart from the cases mentioned above - soluble salts inclusions, volatile components - the accidental formation of defects during its service life is common, that is, in the form of pinholes or scratches. The electrochemical description of a defect next to an intact coating area is shown in Fig. 4. When a small defect is exposed to a corrosive environment - which may be either a bulk liquid phase or only a thin film of condensed water - the part of the substrate that is directly exposed will start to corrode, forming metal oxides and hydroxides that block the defect. These corrosion products are permeable to water but impermeable to oxygen. Therefore, a separation between the cathodic and the anodic areas occurs. Underneath the oxides, that is, at the center of the defect, the anodic reaction takes place, whereas the cathodic reaction occurs further away from the defect [80] (Fig. 6). 

Why does iron continue to corrode even after a layer of rust forms on the surface, whereas aluminum stops corroding after a corrosion layer forms ... 

Tubercniation localised attack in which the corrosion products form wartlike mounds over the corroded areas. 

As corrosion proceeds, the corrosion product formed takes up a larger volume than the steel consumed. This builds up tensile stresses around the rebars. A layer of corroding rebars will often cause a planar fracture at rebar depth, before the concrete spalls, as shown earlier in... 

A second factor is the nature of the scale. Gilbert (1952) showed the very large number of different corrosion products that can form on zinc corroding in water. Schikorr (Table 3.3) gives analyses of corrosion products formed in distilled water or condensates. The nature of the corrosion products in relation to pH and chloride concentration has been shown by Feitknecht (1955) (Fig. 3.2). At water temperatures above about 55°C (Thomas, 1980), the corrosion products that form have a coarse-grained structure and less adhesion to the zinc surface. Corrosion of the zinc will still occur locally as a result of discontinuities in the scale or local electrochemical action. The chemical and electrochemical processes in the formation of the protective layer were discussed by Kruse (1976). 

Examination of a corrosion system experiencing MIC should include (1) metal composition (2) macroscopic examination, (a) visible fouling, (b) localized corrosion, (1) forms, (2) location, (3) material within pits, and (c) corrosion products. Factors of interest in evaluating the environment of a corroding system include the following (1) presence, absence, cycles of light, (2) aqueous medium, (a) temperature. 

Uniform corrosion in a ceramic takes place as a result of chemical dissolution. The corrosion product formed is nonprotective, being either poorly adherent, soluble, or a good transport medium for the corrodent to the ceramic surface. This is a predictable form of corrosion based on test data or experience. 

When a steel nail with zinc plated on the pointed end is placed in the same gel, the iron now acts as a cathode and the zinc as an anode (Fig. 7.42). Accordingly, the red color identifying the cathode develops at the iron half. No iron goes into solution on this area, and, therefore, no blue color develop . The zinc corrodes as an anode at the point and is consumed in providing protection to the iron. A white area develops around the zinc because the zinc corrosion products form a white substance in contact with the potassium ferricyanide reagent. 

Figure 1.44. If reinforcing bars are exposed to moisture and oxygen in a pH neutral environment, they will quickly corrode. Because the corrosion products formed have a larger volume than the transformed iron, the concrete over the reinforcement spalls.

The corrosion potential technique uses the galvanic corrosion potential formed between the exposed and non-exposed surfaces when in a corroding electrolyte. The corrosion potential depends on the area fraction of the exposed surface and the anodic polarization on the exposed surface. For a metal surface covered by a noble metal coating with low porosity, there is a linear relationship between the corrosion potential and the area fraction of the pores. 

Sacrificial protection (corrosion) A form of corrosion protection where one material corrodes in preference to another, thereby protecting it. Examples Zinc and cadmium on steel aluminum on steel. 

Moist iodine vapor rapidly corrodes metals, including most stainless steels. The initial process is the formation of corrosion centers where small amounts of metal iodide are formed which deHquesce, and the corrosion then takes place electrochemically (41,42). Only titanium and molybdenum steels are unattacked by iodine (42,43). The corrosion of molten iodine has also been studied. 

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