Steel will rust, but only in the presence of water, oxygen and Ions. The formation of ferric oxide (rust) requires iron (in steel) and oxygen to combine. This is a reactive process in which chemical change is supported by the flow of electrical current. For the current to flow, ions must be present in the water. Reaction rates vary from area to area and can be much increased by the presence of extra ions in the water, say from acids or salts, that enhance the electrical flow thus increasing the corrosion rate. This is, of course, also governed by the availability of oxygen. For example; with an item partly submerged in the sea, the greatest corrosion rate is between the high and low tide levels. Areas below the low tide level have a restricted oxygen supply. Areas above high tide do not have a consistent supply of water/ions to allow the electrical current flow. Between these levels is rust utopia - plenty of all rust's favourite ingredients and all provided twice a day every day!
It has therefore been necessary to develop corrosion protection methods that successfully combat this unpleasant and costly fact. As yet, however, the best we can achieve is to slow the rate of corrosion thus extending the useful life of the substrate. There are three main methods that have traditionally been used for this purpose, which comprise:
- Barrier coatings. These work in the simplest way, by reducing the amount of the three "rust factors" effect on steel. They may reduce the amount of any one or all three factors to achieve the desired result.
- Inhibitive protection. In general terms, is the use of anti corrosive agents to interfere with the electrical flow at the point of exposure. Similar to barrier protection in that the desired function is to reduce or otherwise alter the electron flow.
- Cathodic protection. This is a more complicated method whereby steel is protected with another material (usually zinc) that protects the steel by corroding in its' place. This method only works with a small range of materials (zinc, magnesium, aluminium etc), as the sacrificial element must be Electro-negative when compared to steel. The coating then becomes the sacrificial anode(-) to the steel cathode(+). Where the coating is Electro-positive compared to steel (chrome, copper etc), the steel becomes the sacrificial element. These coatings may provide some barrier protection but in the event of damage to the coating, the steel will try to protect it! In this case watch out - corrosion in this form tends not to spread under the coating but rather through the substrate leading to perforation and rapid premature failure.
This diagram shows the effect of damage on different types of coatings.
- As can be seen, the cathodic nature of zinc protect the subtrate by setting up a galvanic cell. As the zinc corrodes the precipitation by-products fill the damaged area and continue to protect.
- The barrier type coating is affected with rust starting at the damage site which then continues under the coating causing this to fail also. The coating holds in moisture and assists the spread of rust.
- The Electro-positive coating will give rise to even more rapid corrosion at the point of damage as the steel now becomes the sacrificial element. This will cause the steel to corrode more quickly than if there were no coating. This form of corrosion is more likely to penetrate and perforate the substrate causing premature failure.
Necessarily, none of these different methods will provide all the answers all the time. Indeed there is much merit in combining two or more methods to achieve the desired result. Passivated zinc is a good example - This is zinc plating (cathodic), followed by chromating (inhibitive). With the changes in modern technology it is now possible, with Galv Tech materials, to combine all three methods in a single product for even greater performance.
