Temporary hard-film

There are many temporary protectives on the market and it would be impracticable to describe them individually. However, they may be classified according to the type of film formed, i.e. soft film, hard film and oil film the soft film may be further sub-divided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types. All these types are removable with common petroleum solvents. There are also strippable types based on plastics (deposited by hot dipping or from solvents) or rubber latex (deposited from emulsions) these do not adhere to the metal surfaces and are removed by peeling. In addition there are volatile corrosion inhibitors (V.C.I.) consisting of substances, the vapour from which inhibits corrosion of ferrous metals. 

The total hardness of a given sample of water may be due in part to the presence of bicarbonate and in part to the presence of other soluble salts. When boiled, the normal carbonate is precipitated and, on account of the decrease in solubility of calcium sulphate with rise in temperature above 38° C., there is always a tendency for this substance to separate to some extent with the carbonate. This causes the deposit to form a coherent film on the containing vessel, whereas the pure carbonate gives a more or less powdery suspension. The boiled water is now softer than before, such hardness as it now possesses is termed permanent, whilst its temporary hardness is the difference between the total and permanent hardness, namely, that lost by boiling. 

In normal soft water the corrosion of zinc and zinc alloys is dependent on the oxygen content, which is generally 6-10 mg/L. Hard waters usually produce a protective film, but for water of 2°-6° German hardness, chloride should not exceed 75 mg/L and for water exceeding 6°C German hardness, it should not exceed 150 mg/L if the protective film is to remain sulfate, however, can be substantially higher. Ammonium salts are always unfavorable and should never exceed 20 mg/L. Copper is especially harmful and should be as low as possible, substantially under 0.1 mg/L. At 2 temporary hardness, zinc is attacked below pH 6.8, but in harder water of 2-6 and 7.6-9.6 pH there is no attack if at least 5 mg/L oxygen is present. 

Virtually all end-use formulations of latex polymers of MFT much above 5 °C (40 °F) contain a moderately low volatility solvent for the polymer. These act as temporary plasticizers to ensure low MFT and enhance polymer interdiffusion. The solvent should diffuse and evaporate as quickly as possible for film hardness development, but not at the expense of completeness of film strength development A complication is that compositions of high Tg which contain solvent... 

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