The grain of wood consists of large numbers of interconnected cells with rigid walls of cellulose. The moisture content of wood varies with the humidity of the atmosphere. Changes in moisture content are accompanied by swelling or shrinkage of the cell walls and this results in dimensional changes in the wood as a whole.

Primers for wood, particularly for exterior work must, therefore, possess the following properties:

a) Good adhesion to the surface.

b) Lasting elasticity in order to expand and contract with dimensional changes.

c) Impermeability to moisture.

Older type primers based on white lead and linseed oil are now almost obsolete and replaced by oleoresinous, polyurethane one packs, alkyds and waterbased acrylics.

The pigmentation is commonly titanium oxide with some iron oxide to give the traditional pink colour, with calcium plumbate and aluminium being added to improve moisture resistance and film strength.

Primers resistant to alkaki (present in new cement plasters and asbestos boards) and seal off the porous surfaces. Highly polymerised binders have been found to be the most effective and include chlorinated rubber, tungoil/phenolic, vinyl emulsions and styrene - butadiene copolymer resins. Pigmentation is low because sealer properties are contributed largely by the binder.

Primers for iron and steel are one of the main uses of paint by man through the ages. The natural tendency for structures and fabricated articles of every kind to wear away by the erosive and corrosive effects of the atmosphere.

It has been estimated that the annual cost of corrosion in the UK is of the order of £1,500 million in 1976.

A brief description of the process was given earlier in this module.

It has been established that three conditions are necessary for corrosion to take place at ordinary temperatures, these are:

l) A supply of moisture.

2) A supply of oxygen or air.

3) A potential difference between adjacent areas in the surface of the metal.

The potential difference can occur from:

a) Different iron carbides in the steel crystal grains.

b) Graphite or carbon particles in the steel structure.

c) Broken scale or rust on steel surface.

Broken millscale on the surface-affords a good example of how corrosion can occur. The scale will form a small corrosion cell with the steel in the presence of air and moisture. The steel becomes the anode and scale becomes the cathode. Iron dissolves at the anode, which migrates the the cathode forming rust.

Corrosion of steel is accelerated if the water contains an acid or an electrolyte (e.g. salt) or if the steel is in a stressed condition.

Contact between steel and certain other metals can lead to 'galvanic' or ‘bi-metallic' corrosion, but some metals will actually protect the steel. If steel and copper are in contact in the presence of air and moisture, the steel becomes the anode and the copper the cathode. Under these conditions steel corrodes rapidly.

When in contact with minc, steel becomes the cathode and zinc the anode. Here it is the zinc which corrodes and the steel is 'protected'. The zinc is said to provide ‘sacrificial' or 'cathodic' protection for steel and is used in several forms for this purpose.

The way of knowing which metal will be sacrificially corroded is from knowing the element's standard Redox potential.

Fe (Iron) = - 0.44 E ^o/v

Cu (Copper) = + 0.54 E ^o/v

For this combination, Fe having a more electro-negative potential, will oxidise first.

With Fe and Zn, the Fe = - .44 E ^o/v

          Zn = - .76 E ^o/v

The zinc, having the more electro-negative potential, will form the anode of an electric circuit and corrode (dissolve ions into solution) first.

This information can be calculated from tables of the electro-chemical series, but is accurately gained from standard Redox potentials at 298.15K.