Chromium coatings microcracked

A thickness of at least 0-8 xm is normally needed to ensure that the required crack pattern is formed all over a shaped part. Such microcracked chromium coatings have a slightly lower lustre than the thinner conventional chromium deposits and take longer to deposit. The improved resistance to... 

A potential problem that could result from this system of corrosion control would be the undermining of the chromium and the possibility that the brittle chromium deposits could flake off the surface. This potential problem was prevented by the development of a microcracked or microporous chromium coating. These coatings contain microcracks or micropores that do not detfact from the bright appearance of the chromium. They are formed very uniformly over the... 

Microcracked chromium coatings are produced by first depositing a high stress nickel strike on a sulfur free nickel layer and then a decorative chromium deposit. The uniform crack network results from the interaction of the thin chromium layer and the high stress nickel deposit. The result is a mirror surface as well as a decorative chromium coating. 

Microcracked chromium coatings are resistant to corrosion in the atmosphere. The coatings are not suitable for use in strongly acidic environments. The chromium coatings may not be impressive on their own merit but their contribution in increasing the life of nickel, copper and other coatings is signifrcant. 

How a microcracked chromium coating protects a nickel coating from corrosion ... 

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

The effects of tensile stress in the various layers of nickel plus chromimn coatings are complex, and internal stress in both chromium and nickel (postnickel strike or PNS) layers can be harnessed to produce beneficial cracking ( microcracking ). 

Nickel coatings are usually prepared by electroplating. The metal is plated either directly on steel or sometimes over an intermediate coating of copper. The copper underlayer is used to facilitate buffing of the surface on which nickel is plated, because copper is softer than steel, and also to reduce the required thickness of nickel (which costs more than copper) for obtaining a coating of minimum porosity. The automotive industry uses nickel as an underlayer for microcracked chromium to protect steel [9]. 

To solve this problem, a duplex nickel coating was developed as shown in Figure 16.4. An initial layer of sulfur-free nickel is applied to the steel substrate, followed by an inner layer of a bright nickel containing sulfur, with an outer layer of microcracked chromium. 

For example, a coating on steel of 40 p,m (minimum) bright nickel plus 0.3 xm (minimum) microcracking chromium has the classification number Fe/Ni 40b Cr me. 

The life of nickel coatings may be further increased by a thin overlay of microcracked chromium because corrosion would not penetrate the nickel directly but spread laterally. 

Decide which coating metal will withstand the expected environment and which of the coating metals can be applied to adequate thickness with good coverage. Note The danger of microcracking of thickly applied chromium, rhodium, or hard metals (corrosion rate of deposited metal from economical and technical point of view). 

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