Uniform general corrosion

Additional means of reducing corrosion were the use of prepared steel laths, which were introduced into the FW system and would preferentially rust, leaving boiler tubes unscathed. Sacrificial zinc sheet anodes also were employed, but neither method ensured complete protection of the overall boiler system, and uniform general corrosion was often replaced by insidious localized corrosion. 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

These three assumptions lead to uniform (general) corrosion. But this is only one of several corrosion forms that occur under different conditions. The other forms of corrosion depend on the deviations from the mentioned assumptions. Such deviations... 

The corrosion-like electrochemical process of material removal refers to spatially uniform general corrosion of the metal surface. However, the wet CMP environment can also support certain other types of undesirable electrochemical corrosions, such as localized pitting, and bimetallic/galvanic decomposition that contribute to surface defects. The considerations for mitigating these defects constitute a major aspect of slurry (additive) selection, which in turn can be facilitated by the use of electrochemical techniques. 

The initially apparently contradictory results of the experiments in [19] and [20] must be seen from the point of view of the types of corrosion. [19] deals primarily with uniform general corrosion (removal of iron) and the formation of msty water, while the conclusions in [20] primarily relate to wide or deep pitting corrosion. 

The first three forms of corrosion uniform or quasi-uniform general corrosion, galvanic corrosion and localized corrosion (pitting, crevice and filiform) have no clear separation. The oxide-hydroxide passive layer can play the... 

Pure magnesium should have a driving potential of 850 mV to protect steel but in practice the metal corroded very rapidly with a very low efficiency. The metal suffers low polarization in the presence of chloride or sulfate ions and produce highly soluble chloride and sulfate salts. These ions are usually artificially introduced into the electrolyte as a backfill when a deficiency is expected, the hydroxide which is preferentially formed because of its low solubility becomes enriched with the backfill anions and itself functions as a backfill. Uniform general corrosion can then be obtained and well-designed inserts help to keep most or all of the anode metal available for sacrificial consumption. In freshwater or electrolytes which contain none of these ions, the hydroxide and carbonate may form, but these do not seriously polarize the anode (Morgan, 1993). 

The measurement of weight loss vs. time is the most convenient method to assess corrosion rate. For uniform general corrosion, the corrosion rate is usually proportional to the weight loss and inversely proportional to the area, exposure time, and density of the material. Unfortunately, this method fails to consider the effects of localized corrosion, such as pitting, crevice, and intergranular attack. The corrosive damages due to these localized corrosions can be severe, particularly in accelerated corrosion tests. The different rates of surface film or scale formation between accelerated and natural corrosion tests can further complicate the damage assessment and the correlation between their respective test results. 

The electrochemical process of corrosion is complex, and the corrosion rate depends on myriad physical and chemical parameters. Different degrees of corrosive attack are often observed at different locations on the same part seemingly exposed to the same corrosive environment. Figure 7.6 shows typical uniform general corrosion on a steel tube. Most areas show relatively imiform corrosion, while an unusually severe corrosive attack at one location has penetrated through the tube thickness. Multiple corrosion mechanisms can be activated therefore, different failure modes predominate in accelerated and natural tests. This might require the use of different corrosion models in corrosion rate calculation and introduce different confidence levels in... 

Atmospheric exposure, fresh and salt waters, and many types of soil can cause uniform corrosion of copper aHoys. The relative ranking of aHoys for resistance to general corrosion depends strongly on environment and is relatively independent of temper. Atmospheric corrosion, the least damaging of the various forms of corrosion, is generaHy predictable from weight loss data obtained from exposure to various environments (31) (see Corrosion and CORROSION CONTKOL). 

Reactions with aqueous solutions. Uniform dissolution or corrosion of metals in acid, alkaline or neutral solutions (e.g. dissolution of zinc in hydrochloric acid or in caustic soda solution general corrosion of zinc in water or during atmospheric exposure). Reactions with non-aqueous solution (e.g. dissolution of copper in a solution of ammonium acetate and bromine in alcohol). 

Uniform Corrosion (general corrosion) corrosion in which no distinguishable area of the metal surface is solely anodic or cathodic, i.e. anodes and cathodes are inseparable, cf. localised corrosion. 

General Etch Oxygen Corrosion (Uniform Rate Corrosion/General Wastage Corrosion)... 

FIGURE 22.1 Aspects of metallic corrosion (a) uniform general (b) nonuniform general (c) localized (spots) (d) large pits (e) small pits (/) intercrystalline (g) subsurface. 

General corrosion in the top 5 -inch region of the upper liner after the 500-hour test was also observed. This relatively uniform corrosion is estimated to penetrate the 0.15-inch-thick liner to about 0.008 inch. Had it not been for the pinhole leaks, the liner would have had significant remaining service life. However, Foster Wheeler concluded that the test did indicate the upper... 

Corrosion of metals by aqueous acids with hydrogen evolution is usually rapid and fairly uniform across the surface (general corrosion), since the reductive dissolution of the oxide film that helps maintain the distinction between anodic and cathodic sites is favored by low pH (reaction 16.9). Thus, although local anodic and cathodic areas persist, pitting becomes less important than overall loss of metal. If the oxide film is sufficiently insoluble in acids and is also resistant to reductive dissolution, as with titanium or stainless (>11% Cr) steels, the metal may remain unaffected by aqueous acids, except at quite negative Eh values. In cases where the cathodic discharge of hydrogen ions... 

The corrosion resistance of steels in acids is represented by isocorrosion diagrams. The resistance to uniform or general corrosion in various acids and sodium hydroxide was determined by weight loss measurements over a period of 96 h and the resulting data are given in Table 4.12. 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white mst on zinc on atmospheric exposure are due to uniform corrosion.14... 

The simplest form of corrosion is uniform or general corrosion. This mode of corrosion is characterized by a uniform metal loss over the entire exposed... 

The most common way to report uniform corrosion is in terms of metal thickness loss per unit of time, such as inches per year or millimeters per year. Because uniform corrosion is predictable, even moderately high corrosion rates can be tolerated provided a suitable monitoring and inspection system is utilized. For most chemical process systems, general corrosion rates of less than 2 mils per year (MPY) are acceptable. Rates between 2 and 20 MPY (Imil = 0.001 in.) are routinely accepted as useful engineering materials. In severe environments, rates between 20 and 50 MPY may be economically justified. Rates exceeding 50 MPY are generally not acceptable. 

Metals corrode by an electrochemical reaction where metal loss occurs as oxidation at the anode sites and plating out occurs at the cathode. This phenomenon manifests in two distinct ways general corrosion or localized corrosion. General corrosion refers to a uniform loss of the metal thickness due to the interaction of the metal and the chemical to which... 

Uniform surface corrosion is generally the result of chemical attack by the reaction medium. It can be facilitated by erosion of a passivating layer. A well-known example is scaling of iron surfaces due to oxide formation with hot air. Studies on uniform surface corrosion require test times of 1-2 months, since the attack is more intense in the early stages. The following ranges of corrosion rates can be used for general evaluation ... 

GENERAL CORROSION - Uniform overall corrosion of metal surfaces. 

General corrosion is the most common form of corrosion. This can be uniform (even), quasi-uniform, or uneven. General corrosion accounts for the greatest loss of metal or material. Electrochemical general corrosion in aqueous media can include galvanic or bimetallic corrosion, atmospheric corrosion, stray current dissolution, and biological corrosion (Table 1.1). 

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