Unit 2- Class 7- CORROSION (Corrosion Control)

Inorganic coating or surface conversion coating or chemical conversion coating:

Here a surface layer of the metal is converted into a stable compound by chemical or electrochemical reactions, which forms a barrier between the base metal and corrosion environment. These types of coating are different compared to that of metal coating because they are integral part of the metal itself and in addition to corrosion resistance they also provides increased electrical insulation and also physical appearance of the metal. In surface conversion coating two important type are

  1. Anodized coating or anodizing:
Anodizing usually  carried out in non-ferrous metals like Al, Mg, Cr, Ni, etc or their alloys by anodic oxidation process in which base metal is made as anode, in an electrolytic bath of suitable composition and by passing direct current. However commercially only Al and its alloy are anodized for corrosion resistance.
Process: The article to be anodized is degreased properly and polished. Then made as anode and copper or lead is made as cathode. The electrolyte consists of 5- 10 % chromic acid, the temperature is about 350c.  The voltage is programmed to increase from 0-50V (optimum 40v)
So as to maintain an anode current density of 10-20milli amp/cm2. In first ten minutes the potential is increased from 0-40V and at 40V continue anodizing for about 20min after wards the potential is increased from 40 V- 50V and maintained at 50vc for 5min an opaque oxide layer of 2-8micrometer thickness is obtained. If higher thickness is required 10% sulphuric acid is used as an electrolyte at 220c with around 160milliamp/cm2 of current density at 24V a colorless 25micrometer thickness layer is obtained. Afterwards the article should be dyed properly and finally treated with boil water containing cobalt or nickel sulphate or Acetate to improve corrosion resistance.

Phosphating: It is generally obtained on steel surface by converting surface metal atoms into their phosphate by chemical or electrochemical process.
The phosphating bath containing three essential components 1. Free phosphoric acid 2. Primary phosphate like Fe, Mn, or Mg phosphate. 3. Accelerator such as nitrates, nitrites hydrogen peroxide and PH usually in the range of 1.8-3.2 if the process is immersion type.  If the process is spraying type then PH is in an around 3.2 to 7.8 and temperature maintained at 35oc.

    The mechanism of phosphating involves following steps.
1)    First dissolution of the metal as metal ions.
2)    Metal ions reacting with phosphate ions to form a metal phosphate.
3)    Deposition of the metal phosphate on the surface of the metal.
Phosphating is always given as underline (under coating) before the finishing, because phosphating along with corrosion resistance it also imparts the surface a good paint adhesion quality.

Application:  An undercoating before painting of Automotive bodies, refrigerators, washing machines etc.
     One of most important application of phosphating is of galvanized iron which is otherwise difficult to paint.