Local corrosion metallic coatings

Coatings of more noble metals than the substrate metal (e.g., Cu on Fe) are only protective when there are no pores. In other cases severe local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible. This protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. 

Electrochemical and nonelectrochemical ways to protect metals against corrosion can be distinguished. The nonelectrochemical ways include dense protective films that isolate the metal against effects of the medium and may be paint, polymer, bitumen, enamel, and the like. It is a general shortcoming of these coatings that when they are damaged mechanically, they lose their protective action, and local corrosion activity arises. 

Locus of failure studies 75 80) on metal/epoxy joints that had been exposed to water indicate that corrosion of the metal substrate does not occur until after interfacial failure has occurred. This suggests that corrosion itself does not play a primary role in the loss of adhesion strength mechanism of metal/epoxy joints, but rather is a post-failure phenomenon. However, for the case of metal/epoxy protective coating systems, Leidheiser and coworkers 88-91 -92) and Dickie and coworkers 5 87-89-90> have proposed that localized corrosion processes are part of an important delamination mechanism. 

The high electrical resistivity of aluminum oxide is believed to be the major reason why coatings continue to exhibit very strong adhesion to aluminium substrates even when localized corrosion is observed to occur. Therefore, by developing a pretreatment process for any metal substrate which produces a metal oxide with high electrical... 

Coatings and localized corrosion. This type of corrosion can occur where protective coatings are applied over metal and where there is a break in the coating so that the large coated area acts as a cathode, and the small defective area as the anode.26... 

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

Any system Small corrosion area or general corrosion over large area Localized corrosion Evaluate structural integrity. If fit for service, then apply coating to protect metal and inspect according to schedule Identify root cause of corrosion. Remove materials causing corrosion. Replace damaged material... 

Figure 10.22 Localization of corrosion at a defect in a metal coating on steel, a) Cathodic coating, b) Anodic coating.

An example of localized corrosion in the form of pits in natural protective coatings is the corrosion that arises through cracks in the rolling scale of unalloyed or low-alloy steels. If, for example, the rolling scale of a steel plate has spelled at a tiny point, and the sheet is immersed in a salt solution containing oxygen, an electric current flows between the oxide film (the cathode) and the bare metal (the anode). At the anode, iron dissolves as iron(II) ions, whereas at the cathode, oxygen is reduced ... 

If different metals are used in a PU electrode pair, the different half-cell potentials may easily create a DC voltage output of 1 V or more. This may represent a source of noise and may saturate the input stage of the biopotential amplifier used. Inhomogeneous surfaces may create local potential differences, local DC current flow, and local corrosion, even with zero current flow in the electrode wires. If an AgCl surface is partly stripped its coating so that some pure silver surface appears, the DC voltage will change and the noise level will increase (see Table 7.2 for half-cell potentials). 

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