TPi April 2010

outer surface. Scratches which do not disappear after electroplating are seen in figure 2.

The so-called decorative chrome plating process (also referred to as nickel-chrome plating) differs from industrial (hard) chrome application as the thickness of chromium deposit layer is below 1µm. Under this most outer chromium layer, there exists deposits of nickel and copper at higher thickness values, between 10 to 25µm. Technical requirements for making tubes suitable for decorative painting Electroplating is different from painting applications as it has the ability to fill in the scratches and pits to a certain extent. Surface preparation of the substrate to be painted is very important. As high as 80% or more of all coating adhesion failures can be directly attributed to improper surface preparation. The most common forms of surface debris are oils or greases that originated from mechanical processing or are deliberately applied for purposes of corrosion prevention during temporary storage or shipping. Other surfacecontaminants commonly includeoxidation, rust, corrosion, heat scale, tarnish and, in some cases, old paint. Dirt, grease, or other similar materials will block the bonding surface and create an imperfection on the finished part. Sand blasting, shot blasting and mechanical cleaning are the commonly referred methods to prepare the steel surface for painting. While there is no universally acceptable reference for evaluating the conformity of steel surfaces to be suitable for painting applications – based on numerical criteria – the international standard ISO 8501 “Preparation of steel substrates before application of paints and related products – visual assessment of surface cleanliness” is a universal reference for this purpose.

Figure 2 : Scratches that do not disappear after electroplating

surface preparation and finishing operations (grinding, polishing, etc) following proper process steps. The key point is to grind and polish the surface to maintain a “mirror finish” condition, to an extent that the surface roughness will be low enough. Ra<0.2µm has to be maintained uniformly all around the outer surface.

It may be necessary to remove an amount of 30µm from the outer surface, by the use of mechanical abrasives.

The amount of metal that has to be removed from the outer surface of steel tube, by means of grinding, is shown in figure 1. The use of grinding media rougher than 240grit is not recommended. The electroplating process does not make surface defects such as scratches and pits disappear by filling in. The deposit just follows the contour of the tube’s

Figure 3 : Layers of nickel, copper and chromium deposit achieved by electroplating

Figure 4 : Typical values for the thickness of plating element layers on steel tube

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Tube Products International April 2010

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