Pitting localized

In the following, the most typical modes of corrosion—other than the above discussed unifonn dissolution (active corrosion) and localized pitting and crevice corrosion (local active dissolution)—are briefly presented. 

Concreteyiggregate. A versatile appHcafion for coarse limestone aggregate is in Pordand cement concrete, either job-mixed or ready-mixed for a wide variety of concrete appHcafions, such as footings, poured foundations, paving, curbs, stmctural products, etc. Limestone sand also provides a satisfactory fine aggregate, but usually it is more cosdy than conventional sand from local pits. 

The CBD diagram can provide various lands of information about the performance of an aUoy/medium system. The technique can be used for a direc t calculation of the corrosion rate as well as for indicating the conditions of passivity and tendency of the metal to suffer local pitting and crevice attack. 

Wastage may appear as general attack or as grooving, or it may be localized. Pitting almost never occurs. Each attack morphology is characteristic of concentration processes associated with corrosion. 

Nitrite-based programs require a relatively high application rate to ensure that all anodic areas within the system are fully protected from the risk of pitting corrosion. Undertreatment exposes anodic areas, which are subject to localized pitting as a result of the concentrating power from surrounding cathodic sites. 

A plot of mass loss versus time can provide information about changes in the conditions under which the test has been run. One example of such a plot comes from the ASTM Standard G96, Standard Guide. As mentioned previously, weight loss measurements are appropriate for measurement of localized pitting corrosion, including that caused by MIC. 

The importance of MIC has been underestimated, because most MIC occurs as a localized, pitting-type attack. In general this corrosion type results in relatively low rates of weight loss, changes in electrical resistance, and changes in total area affected. This makes MIC difficult to detect and to quantify using traditional methods of corrosion monitoring [1447]. 

The ionic phosphonates like NTMP are effective hydration inhibitors because they can form an insoluble complex with the oxide surface. They are useful as epoxy adhesive couplers in cases where the adhesive and its curing cycle are compatible with the adsorbed phosphonate molecule. (14) Wedge test results indicate that in two epoxy-aluminum systems studied, certain organosilanes tend to both increase the epoxy-metal bond durability and maintain hydration resistance. The results of anodic polarization experiments further suggest that these silane films are effective against localized pitting. 

Also, undertreatment in this type of system is not recommended because of the continual exposure of new metal whenever rust is removed. If inhibitor is not present in adequate amounts, localized pitting-type corrosion will continue. 

One effect is that bio-fouling of the metal surface often promotes localized attack. When a barnacle or mussel attaches itself to a plate of stainless steel in sea water, a differential aeration corrosion cell is formed. Intense local pitting results which may lead to complete perforation in a relatively short time. In one case a 0.75-inch thick stainless steel plate was perforated beneath a barnacle base after 9 months of immersion in sea water. 

The main advantage of this group of materials is resistance to pressure. However, there are also disadvantages, such as a high rigidity in comparison to bone, local pitting corrosion due to the fact that there is no protective layer, metal fatigue and friction corrosion. The latter results in rejection which is caused by released ions. 

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

The corrosion rate of ductile iron pipes in soils was in the range 0.62-2.5 mm/yr with an average of 1.11 mm/yr which is twice the rate of cast iron pipe. Failure of ductile iron pipe was solely due to localized pitting corrosion leading to pipe wall perforation. 

Anodic inhibitors reduce corrosion by polarizing the anodes rapidly. If insufficient anodic inhibitors are used, severe localized pitting will occur in the unprotected areas. Examples of anodic inhibitors are phosphates, silicates, nitrates, and chromates. ... 

The ability of the ferritic alloys to resist chloride see is one of their most useful features in terms of corrosion resistance. During the 1970s, developmental efforts were directed at producing ferritic materials that could also exhibit a high level of general and localized pitting resistance as well. 

The localized pitting-type corrosion of carbon steels can generally be attributed to one or more of the following ... 

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