Grain attacks

Henthorne , in considering the corrosion testing of weldments, points out that the test will also give high rates due to (a) end-grain attack, which is particularly prevalent in resulphurised or heavily cold-worked material and (d) dissolution of Ti(C, N) such as occurs in Type S2J weldments and leads to knife-line attack. Since most service conditions do not cause attack on the alloy in these conditions the test can be misleading. 

Phenomenology of corrosion for metals and polymer materials is not the same. For example, the terms such as corrosion rate, pitting, end grain attack, and intergranular attackusQd to describe metals behaviors are not applicable to plastics. Not only are the terms for plastics different blistering, discoloration, cracking, etc.) but they are based on a naked-eye observation and not at a microscopic level as for metals. 

A number of participants reported corrosion along the outer rim of the coupons. This would be expected from end grain attack on cut surfaces. 

Corrosion test specimens are used to evaluate average corrosion rate over the exposure period and eire also useful for assessment of crevice corrosion, pitting, and end grain attack, and may be used for metallographic examination of the corrosion test specimen or analysis of any deposits. Special corrosion test specimens may be prepared with welds to assess corrosion problems particular to weld material or heat affected zones. SCC may be monitored with specially mounted and loaded corrosion test specimens. 

Nearly 180 trichothecenes and their derivatives are currently known. While trichothecenes of types C and D do not commonly occur in crops, trichothecenes of types A and B are widespread. The presence of one and sometimes more trichothecenes has been demonstrated mainly in cereals (wheat, barley, oats and maize) grown in the temperate zones of Europe, America and Asia. Grains attacked by Fusarium spp. are characterised by a typical reddish colour. Findings of trichothecenes A and B have also been reported in soybeans, oil seeds, banana and mango. 

To calculate the still free area Si, for nucleation at t time, we indicate by N the number of grains not yet attacked at t time. A grain attacked at rtime is immediately covered with a film of solid B the free area for nucleation will be given by the product of the initial area of a grain and the number N of not attacked grains... 

Fractional extent at t time of a grain attacked at t time. 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Sometimes the formation of oxide films on the metal surface binders efficient ECM, and leads to poor surface finish. Eor example, the ECM of titanium is rendered difficult in chloride and nitrate electrolytes because the oxide film formed is so passive. Even when higher (eg, ca 50 V) voltage is apphed, to break the oxide film, its dismption is so nonuniform that deep grain boundary attack of the metal surface occurs. 

Corrosion is a complex phenomenon tnat may take any one or more of several forms. It is usually confined to the metal surface, and this is called general corrosion. But it sometimes occurs along grain boundaries or other lines of weakness because of a difference in resistance to attack or local elec trolytic action. 

Intergranular Corrosion Selec tive corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

Identification. Weld decay characteristically produces distinct, narrow zones of disintegration immediately adjacent and parallel to the weldment (Fig. 15.4). Attack will occur on either side of the weld. The weld itself and surrounding metal will be unattacked. The corroded area frequently has a granular or sugary appearance and feel. This is due to corrosion along the boundaries of individual grains that causes them to be released from the metal surface. Affected metal may have lost its metallic ring. 

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