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Fazlutdinov K.K.
21.12.2017 (updated 29.01.2018)
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Auto tuning and restoration

Content:

 1. Chrome plating in auto tuning.

2. Restoration of parts on vintage cars.

During the operation of our enterprise, we have repeatedly received applications for chrome plating of various car elements or for their restoration. In this regard, I would like to write a few words on this topic. But first, we advise you to look at the articles "Metallization of dielectrics" and "Chromium plating at home".

 1. Chrome plating in autotuning.

Let's define   how. Chrome is a metal. This is an important note, because, as will be shown below, not all the "chrome" that the market offers motorists is metal. Not everything is chrome that glitters. In addition, chrome plating — it is not a common name for all galvanic processes. In addition to chrome plating, there is also nickel plating, zinc plating, etc. (see relevant sections). And yet - decorative chrome has one single silver-bluish color. Occasionally — black. If you are offered chrome in all colors of the rainbow, you are most likely being deceived.

Why is chrome the go-to for decorative car trim? Everyone knows the Soviet chrome bumpers on the Zhiguli or Volga. So, in these and other products of the Soviet automobile industry, not pure chromium was used, but a three-layer copper-nickel-chromium coating with electrochemical polishing. We will not go into the details of copper plating, because. the copper coating also consisted of two layers, which was due to the impossibility of copper-plating steel from a standard sulfate electrolyte.

So what's good about three-layer coating?

Firstly, shiny chrome can only be deposited over a shiny underlayer. This feature, to a greater or lesser extent, is inherent in all electroplating. But for chromium, it is characteristic in the highest degree. Therefore, a shiny underlayer of nickel is required, which, in turn, is better deposited on copper. Such a sandwich.

Secondly, the deposition of thick chromium is very problematic (for electroplated coatings, a thickness of 0.030 mm is already quite large, if we do not take into account electroplating with coating thicknesses of 1 mm or more), i.e. To. layer-by-layer build-up is required with intermediate polishing and heat treatment of each layer. Today, this problem is solved by electroplating, which uses simultaneous coating deposition and mechanical processing. And for the time being it is exotic for Russian electroplating. Therefore, chromium is usually deposited in a thickness of 4-10 micrometers. Due to its porosity, chromium does not provide corrosion protection for steel products. Therefore, before chromium, it is necessary to apply an undercoat, as a result of which one coating closes most of the pores of the other and the steel base is better isolated from the corrosive environment.

Thirdly, there is the concept of anode and cathode coating. What does it mean? There is a standard range of metal potentials in which the potential can be positive or negative. In the case of electroplating, two different metals come into contact with different potentials. If the coating has a more negative potential than the base metal (in our case, steel), then it will act as an anode, i.e. if there are pores in the coating or damage in which the corrosive medium has entered, the coating will be destroyed, and not the base.

Such a long digression was necessary in order to understand what is offered to us today under the names "chrome", "meta-chrome" and "thermochrome". Meta-chromium plating or catalytic chromium plating is the process of obtaining the thinnest layer of silver (thousandths of a millimeter) on a product by the reaction of a silver mirror (recovery of silver from an inorganic salt). A layer of silver is applied over the primer and protected by various varnishes, which give this "chrome" color. Given that silver is 11 times softer than chromium, it cannot be recognized as a wear-resistant coating. This is typically a cathodic coating and will therefore accelerate corrosion of the steel at the site of damage. And given that the meta-chromium coating is very thin, one should not expect any significant protective properties from it. So, on a meta-chromed part, you can easily write your name with a cutter (on a chrome finish, the cutter will most likely just become dull). Thus, all the "chrome" that a car enthusiast has to deal with today is highly reflective varnish. The first good sharp stone on the road - and a scratch.

Why doesn't anyone make electroplated (real) chrome? There are several reasons for this too.

  1. High energy intensity of the process. Chrome plating of a bumper requires up to 30 kW of electrical power and kiloampere current rectifiers.
  2. Large dimensions of the bath. Given the dimensions of the details of the bath should be about 1000l
  3. Expensive equipment - a modern 900A FlexCraft switching rectifier costs more than 600,000 rubles. And that's not counting all the other equipment.
  4. Extremely low scattering power of the electrolyte. This means that only cylindrical parts can be chromium-plated evenly in a standard galvanic installation with plane-parallel anodes. For more complex shapes of products, it is necessary either to change the shape of the anodes, making them equidistant at any point from the product, or to introduce additional anodes, otherwise chromium will not lie in recesses or bends at all.
  5. Great environmental damage, both in terms of the atmosphere of the workshop and in terms of wastewater.
  6. Normal adhesion of the coating can be obtained on steel products, i.e. on stamped discs. Chrome-plating cast aluminum wheels is extremely difficult, because. the laborious process of chromium plating is combined here with the no less laborious process of aluminum nickel plating (a capricious operation of zincate processing appears) 

2. Now let's deal with the restoration of parts on rare cars.

Restoration consists of removing three layers of coating, soldering corrosion sores with tin and coating with copper-nickel-chromium. Consider them in order. 

  1. Uncovering. If we are dealing with a technical, and not a decorative product, then the removal of the old coating, in principle, will not be a big problem — sufficient sandblasting. However, in the case of decorative finishes, this method is limitedly applicable, because. creates a significant surface roughness, and it will no longer be possible to obtain a mirror galvanic coating without significant polishing. Removing the old coating by chemical means involves difficulties of a different kind. Chemical removal of chromium in hydrochloric acid does not cause problems, but nickel removal will have to be tricky. Here, either a chemical method, associated with expensive organic substances, or an electrochemical method, associated with a significant energy intensity, is applicable. If we are dealing with a damaged part, then when the coating is removed from it, chemical damage to the steel is possible in a place where the old coating has already corroded or peeled off.
  2. After removing the old coating, you need to remove traces of corrosion — dimples and ulcers. For this, tin sealing is used, which also has the disadvantage of — so-called. "tin plague". Tin plague — this is a phenomenon of a polymorphic transition of white tin to gray, which has the appearance of a powder. In other words - "destruction" of tin under the influence of low temperatures. This process occurs most actively at a temperature of minus 33 degrees Celsius. Given the winter frosts in Russia, we can say that damage to the solder at this time is very likely. The problem is solved by adding bismuth to the tin alloy.
  3. It has already been written about applying a new chrome coating.
End of article
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