Corrosion types uniform

The following criteria are usually applied when analyzing these power spectra the magnitude of power, the frequency at which the power begins to fall off, and the slope of the descending part of the plot. From such an analysis one can sufficiently well identify the various types of corrosive attack (uniform, crevice, pitting). 

The two basic types of corrosion discussed above form the general background to the subject. How, and to what extent, any particular object or structure corrodes also depends on other factors, in particular, on whether corrosion is uniform or not and on the effects of mechanical strain. These factors are interactive and in combination, their individual effects can be enhanced. 

Single-component corrosion types, important for heat exchanger design and operation, are as follows (1) uniform attack corrosion, (2) galvanic corrosion, (3) pitting corrosion, (4) stress corrosion cracking, (5) erosion corrosion, (6) deposit corrosion, and (7) selective leaching [153],... 

General Corrosion, or Uniform Attack. This type of corrosion includes the commonly recognized rusting of iron or tarnishing of silver. Fogging of nickel and high-temperature oxidation of metals are also examples of this type. 

All forms of corrosion must be considered at the beginning of any test program, before discounting attack modes that are not likely to occur. Corrosion types can be divided into general (or uniform) attack and localized corrosion, in which extensive attack can occur in a very small area. Localized corrosion can be more difficult to observe. 

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. 

The type of corrosion, if there is corrosion, is detected visually. If the corrosion is uniform, the dissolution rate is determined by measuring the mass loss. This kind of test has evaluated the resistance of aluminium in many media, and the reported results are summarised in Parts E and F. 

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). 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

The condition of the test metal is important. Clean metal samples with uniform finishes are preferred. The accelerating effects of surface defects lead to deceptive results in samples. The ratio of the area of a defect to the total surface area of the metal is much higlier in a sample than in any metal in service. This is an indication of the inaccuracy of tests made on metals with improper finishes. The sample metal should have the same type of heat treatment as the metal to be used in service. Different heat treatments have different effects on corrosion. Heat treatment may improve or reduce the corrosion resistance of a metal in an unpredictable manner. For the purpose of selectivity, a metal stress corrosion test may be performed. General trends of the performance of a material can be obtained from such tests however, it is difficult to reproduce the stress that actually will occur during service. 

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... 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

With a single-phase brass the whole of the metal in the corroded areas is affected. Dezincification may proceed fairly uniformly over the surface, and this layer type takes much longer to cause perforation than the localised plug type that more often occurs . With a two-phase brass the zinc-rich 8 phase is preferentially attacked as shown in Fig. 4.12. Eventually the a phase may be attacked as well. The zinc corrosion products that accompany dezincification may be swept away, or in some conditions may form voluminous deposits on the surface which may lead to blockages, e.g. in fittings. 

Safe or dangerous inhibitors Each inhibitor must be present above a certain minimum concentration for it to be effective (see Principles), and this classification relates to the type of corrosion that will occur when the concentration is below the minimum, or critical, value. Thus, when present at insufficient concentration a safe inhibitor will allow only a uniform type of corrosion to proceed at a rate no greater than that obtaining in an uninhibited system, whereas a dangerous inhibitor will lead to enhanced localised attack, e.g. pitting, and so in many cases make the situation worse than in the absence of an inhibitor. 

---