Tinning is the deposition of a layer of tin or a tin-based alloy on a metal surface. Tin coating is used to protect metal products from corrosion. Its main advantages are good adhesion and corrosion protection, as well as easy machinability.
There are two types of tinning: hot and electroplating.
Hot tinning is the oldest method of applying tin to items either by immersion or by rubbing. The hot method is called because the product is processed with molten tin, which has a melting temperature of 232C.
Plunge tinning involves treating the product in a bath with zinc chloride, which acts as a flux, and then moving it to a bath with molten tin at a temperature of 270-300C.
Rubbing tin is familiar to everyone who has held a soldering iron at least once. First you need to apply a layer of flux on the metal surface with a heated soldering iron. Then transfer a certain amount of solder to the metal and use the hot soldering iron to smooth it out on the surface. Our fathers and grandfathers used the rubbing and tinned by this method products have not rusted for 30-40 years.
There are some things to keep in mind during tinning. Tin is a polymorphic metal, i.e. it can have a different crystal lattice under different conditions. Under normal conditions it exists in the form of β-modification, called "white tin". This is exactly the tin we are dealing with. This form is stable at temperatures above 13.2C. At low temperatures, the crystal lattice begins to change and the tin transitions to another form called "gray tin. This transition is accompanied by an increase in volume, which can lead to the destruction of the tin coating. This phenomenon is called the "tin plague.
How to avoid this? It turns out that a small (less than 5%) addition of bismuth to the alloy can help prevent the conversion of white tin to gray.
Another thing is that when tinning steel surfaces, tin and iron can form galvanic pairs. Here again we are faced with the electrochemical problem. The higher standard reduction potential, the more strong oxidizing agent the metal is. In the series tin, iron, zinc, the strongest oxidizer will be tin with a standard potential of -0.136V, then iron (-0.441V) and zinc (-0.761V). That is, if you cover the entire surface of iron with a layer of tin, it will be a coating that will last for many decades. If you think of patching up a leaky steel pan with tin, you will only open the way to electrochemical corrosion.
Tinning large parts will require zinc chloride, a blowtorch, loose solder, a brush and a cloth for wiping.
In the automotive industry, tinning used to protect welds from corrosion, but with the introduction of spot welding, tinning is no longer necessary. It is used in the restoration and repair of older vehicles now. Tinning is used to restore welds or to repair minor damage to the body if the tinker prefers troweling over puttying.
Galvanic tinning is used to tinning products of various shapes and sizes and is carried out in an alkaline or acidic electrolyte. The process is based on an electrochemical reaction in which tin ions from the electrolyte are transferred to the surface of the product. This ensures a very reliable bond between the tin layer and the product, a uniform thickness of the protective layer and its low porosity, which, in turn, ensures good performance characteristics of the coating.
