Zinc coatings: electroplated or not?

Currently, more than 13 million tons of zinc - 70 % from ore, rest 30 % – from returnable scrap. More than half of this amount goes to zinc coatings to protect steel from corrosion..

Zinc, like all metals, corrodes when exposed to the atmosphere.. However, due to its ability to form dense, well-adhered corrosion products, zinc corrosion rate is much lower, than steel - from 10 to 100 times less depending on the medium.

Barrier corrosion protection

Zinc corrosion products form a kind of additional protective coating on the surface of the zinc coating.. Often referred to as "zinc patina". The zinc patina acts as an additional barrier between the steel and the outside environment..

Cathodic corrosion protection

In addition to the natural barrier protection of coating and patina, zinc provides cathodic protection to the steel.. Known, that metals can be arranged in galvanic rows of metals according to their electrochemical potentials in various environments, eg, galvanic series of metals in a salty environment. When two metals join, one of them, which is higher in the list becomes anodic and corrodes first, thereby protecting the metal, which the, located in the galvanic row below it. Therefore, zinc is anodic in relation to steel and “sacrificially” corrodes to protect steel., on which it is located.

Various zinc coatings

There are several methods for obtaining zinc coatings. (picture):

  • spraying;
  • hot dip galvanizing;
  • painting with zinc paints;
  • continuous hot dip galvanizing of sheets;
  • electrolytic galvanizing.

cinkovye-pokrytiyaFigure - Microstructure of various zinc coatings
(Source: galvanizeit.org)

Zinc coatings, obtained by various methods, differ in thickness, microstructure, bond strength to steel, hardness, corrosion resistance, service life, as well as cost.

Degree of galvanizing: specific gravity or thickness?

The degree of galvanizing of steel is best set by the amount of mass of zinc per unit area of ​​coating, than the coating thickness. It is the amount of pure zinc on the coating that determines the chemical, cathodic protection of steel. for instance, our GOST 9.307 provides for the control of the minimum thickness of hot zinc coating by magnetic or metallographic method, and American ASTM A 153 – minimum specific gravity or coating thickness. Earlier editions of ASTM A 153 give a simple relationship between the specific gravity and thickness of a hot zinc coating: 1 g / m2 corresponds 0,141 µm average coating thickness. In other words, as specified in the standard (!) – seven coating thicknesses in micrometers are approximately equal to the mass of the coating in grams per square meter.

What is in general galvanic coating?

All zinc coatings in the English literature are united by one term "galvanizing", although each of them has its own characteristics. It unites them, that they are all zinc and all protect the steel "galvanically", that is, they provide it with cathodic protection. "Galvanized steel" in English will be "galvanized steel". true, American zinc paint coating is still not commonly called galvanized (ASTM A 902).

According to GOST 9.008-82 the term "plating" has a narrower meaning – it is "electrochemical coating, obtained by cathodic reduction of metal". According to this definition, electrochemical zinc coating is electroplated, and hot zinc - no more. However, GUEST 9.303 uses this term – "electrochemical zinc plating".

As for me, name all zinc coatings – and hot, and cold – galvanic, according to their main function, quite logical. Especially, what for coatings, obtained by electrical methods, there are spare names - electrolytic, electrochemical and even electrodeposited. So the question "What is electroplating?'I would leave open.