Comparison of galvanized and painted steel

Painted Steel

One well developed coating for steel is paint. Numerous types of paint have been developed through the years that offer excellent performance on steel sheet. Paints are barrier coatings that, when applied and used properly, give sufficient corrosion protection to steel for many common applications. However, paints are not impervious to moisture and rust can occur underneath even a perfectly applied paint if the exposure time to moisture is sufficiently long, or at shorter times if the moisture contains corrosive chemicals. Also, exposed areas on the steel, such as sheared edges and scratches through the paint, are susceptible to the same rusting mechanisms as unpainted steel. Further rusting in these exposed areas can cause degradation of the paint adhesion. For example, the iron oxide that forms at a scratch lifts the paint film immediately adjacent to the scratch. This allows further under-film corrosion of the steel and the eventual loss of paint adhesion. Some paints are better at resisting this than others, but eventually, if there is sufficient moisture present, and especially if the moisture contains highly corrosive chemicals, unacceptable levels of rusting will occur. Even if the amount of rust does not adversely impact the strength of the steel, it does lead to an unsightly appearance.

Galvanized Steel protect steel in two ways:                                
1. like paint, they provide barrier protection, and                                
2. galvanic protection in most instances.                                
Detailed information about barrier & galvanic (cathodic) protection is given in link advanced info.