Corrosion uniform

Uniform corrosion occurs when corrosion is quite evenly distributed over the surface, leading to a relatively uniform thickness reduction. Metals without significant passivation tendencies in the actual environment, such as iron, are liable to this form. Uniform corrosion is assumed to be the most common form of corrosion and responsible for most of the material loss. However, it is not a dangerous form of corrosion because prediction of thickness reduction rate can be done by means of simple tests. Therefore, corresponding corrosion allowance 

Uniform attack or general corrosion is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or a very large area. This mechanism has not been a significant concern with spent nuclear fuel in wet 

General or uniform corrosion, as found in other metals, is not to be expected in the stainless steels. The many sets of corrosion data and charts found in the literature that show various corrosion rates of stainless steel in certain environments are actually indicating that the stainless alloy, imder those conditions, is fluctuating between an active and passive condition with a net result of so many mils per year loss. These may or may not be reliable figures. Consequently, recommendations should be based on rates of less than 5 mpy and preferably less than 1 mpy. Under these conditions, no corrosion allowance need be specified. 

General Description. Uniform or general corrosion, as the name implies, results in a fairly uniform penetration (or thinning) over the entire exposed metal surface. The general attack results from local corrosion-cell action that is, multiple anodes and cathodes are operating on the metal surface at any given time. The location of the anodic and cathodic areas continues to move about on the surface, resulting in uniform corrosion. Uniform corrosion often results from atmospheric exposure (especially polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. 

Prevention. Uniform corrosion can be prevented or reduced by proper materials selection, the use of coatings or inhibitors, or cathodic protection. These corrosion prevention methods can be used individually or in combination. 

This term describes the more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack. If the corrosion of a material can be considered to be uniform the life of the material in service can be predicted from experimentally determined corrosion rates. 

Corrosion rates are usually expressed as a penetration rate in inches per year, or mills per year (mpy) (where a mill = 10 3 inches). They are also expressed as a weight loss in milligrams per square decimetre per day (mdd). In corrosion testing, the corrosion rate is measured by the reduction in weight of a specimen of known area over a fixed period of time. 

When judging corrosion rates expressed in mdd it must be remembered that the penetration rate depends on the density of the material. For ferrous metals 100 mdd = 0.02 ipy. 

What can be considered as an acceptable rate of attack will depend on the cost of the material the duty, particularly as regards to safety and the economic life of the plant. For 

The corrosion rate will be dependent on the temperature and concentration of the corrosive fluid. An increase in temperature usually results in an increased rate of corrosion though not always. The rate will depend on other factors that are affected by temperature, such as oxygen solubility. 

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Indium chemicals and electroplated metal deposits ate replacing mercury (qv) in the manufacture of alkaline batteries (qv). Indium, like mercury, functions to reduce outgassing within the battery and promotes the uniform corrosion of the anode and cathode while the battery is under electrical load. Indium inorganic chemicals also find use as catalysts in various chemical processes. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

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). 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

It is usually hmited to the measurement of uniform corrosion only and is not generally satisfactory for locahzed corrosion. 

The assumption of uniform corrosion is also at the heart of the measurements made by the electrical resistance (ER) probes. Again, ASTM Standard G96 outhnes the method for using ER probes in plant equipment. These probes operate on the princi e that the electrical resistance of a wire, strip, or tube of metal increases as its cross-sectional area decreases ... 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

It is a consequence of the action of different pH values in the aeration cell that these cells do not arise in well-buffered media [4] and in fast-flowing waters [5-7]. The enforced uniform corrosion leads to the formation of homogeneous surface films in solutions containing Oj [7-9]. This process is encouraged by film-forming inhibitors (HCOj, phosphate, silicate, Ca and AP ) and disrupted by peptizing anions (CP, SO ") [10]. In pure salt water, no protective films are formed. In this case the corrosion rate is determined by oxygen diffusion [6,7,10]... 

Uniform corrosion is the deterioration of a metal surface that occurs uniformly across the material. It occurs primarily when the surface is in contact with an aqueous environment, which results in a chemical reaction between the metal and the service environment. Since this form of corrosion results in a relatively uniform degradation of apparatus material, it can be accounted for most readily at the time the equipment is designed, either by proper material selection, special coatings or linings, or increased wall thicknesses. 

Avoid materials that crack or pit uniform corrosion is safer than nonuniform corrosion patterns. 

Corrosion may take various forms and may combine other forms of damage (erosion, wear, fatigue, etc.) to cause equipment failure. The forms of corrosion most encountered in drilling equipment are uniform corrosion and galvanic corrosion. 

Ail homogeneous metals without differences in potential between any points on their surfaces are subject to this type of general attack under some conditions. Uniform corrosion is usually characterized by a chemical or electrochemical attack over the entire exposed surface, Figure 4-423. Metal corrodes in an even... 

In this section the interaction of a metal with its aqueous environment will be considered from the viewpoint Of thermodynamics and electrode kinetics, and in order to simplify the discussion it will be assumed that the metal is a homogeneous continuum, and no account will be taken of submicroscopic, microscopic and macroscopic heterogeneities, which are dealt with elsewhere see Sections 1.3 and 20.4). Furthermore, emphasis will be placed on uniform corrosion since localised attack is considered in Section 1.6. 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

It is now appropriate to apply the above considerations of the operation of a well-defined electrochemical cell to the uniform corrosion of a metal in a solution of high conductivity, and under these circumstances both IR and 7/ so, may be regarded as negligible. Thus E will tend to zero, and Ep will tend to be equal to E (within 1-2 mV)... 

Fig. 1.46 Localised attack due to discontinuity in millscale or a deposit at a steel surface, (a) Uniform corrosion, (b) localised attack at a discontinuity in millscale, (c) increase in rate due to increase in the supply of oxygen and (d) crevice formed by deposit. (Arrows pointing downwards represents the cathodic current, and arrows pointing upwards the anodic current)...

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Visual examination of the immersed samples showed that ZA8 suffered moderate, uniform corrosion in both tanks. ZAI2 showed extensive localised corrosion in the primary tank but slight uniform corrosion with localised corrosion at several spots in the secondary tank. ZA 27 suffered extensive localised corrosion over most of the surface in the primary tank but only slight uniform corrosion with several very small pits dispersed all over the surface in the secondary tank. 

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. 

Rp data are meaningful for general or uniform corrosion but less so for localized corrosion, including MIC. In addition, the use of the Stem-Geary theory where the corrosion rate is inversely proportional to Rp at potentials close to is valid for conditions controlled by electron transfer, but not for the diffusion-controlled systems frequently found in MC. 

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