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How Rust Prevention Coatings Work
Understanding Rust Prevention
Let's Talk About Rust!
To understand how rust prevention coatings work, first we need to talk a little bit about rust. Rust is a chemical process in which the iron atoms combine with oxygen atoms to form iron oxide, otherwise known as rust. In a little more detail it is an electrochemical reaction. The iron (or steel) is in an electrolyte (water) and a tiny battery is formed whereby elctrons flow from the iron to the water molecules. As a result, the iron goes from an iron atom to being a ion, which is an atom which has lost one or two electrons. At the same time the water is changed into hydroxly ion (OH one oxygen and one hyrdrogen atom) which gains an electron and it also needs an extra Oxygen atom. The Iron ion then seeks out the hydroxyl ion, they come to gether and form a hydrated iron oxide compound. When this dries out, it becomes the red crumbly ferric oxide (rust) we are so familiar with. It does not need much water at all for this to happen, it can be a drop, a film of water, even just atmospheric humidity or it can be full immersion. In the real world it is a bit more complicated, but not much.
Next we need to understand what is possible and what is sensible. All iron (steel) will eventually return to the rust condition (which is what iron ore is). Rust control is all about delaying this happening. The better treatments can delay it for a long time, hundreds of years and more. Just about any treatment will provide a bit of a delay. How quickly it happens is controlled by how aggressive the conditions it is exposed to are and how effective the control solution is.
To control rust, it is necessary to intervene in one of more steps in the electrochemical pathway from happening.
There are many different ways to control rust, some more successful than others and as a result there is any amount of snake oil claiming to be "Grandpa's Patented Rust Remedy" many of which have little evidence as to their longevity or efficacy. As a general guide, anything which does not explain how it works, probably doesn't work all that well. The field is also full of potions and brews, some from large and respectable paint companies, which by using a brand name imply that the product will kill rust or do other wonderful things. Any coating will restrict rust a bit, but only properly formulated coatings with ingredients scientifically validated to be effective against rust are likely to give significantly extended life and a superior anti-rust experience.
Sacrificial Coatings
The idea here is to understand that rust is a little bit like a battery. There is a cathode which supplies elctrons and there is an anode which absorbs electrons from the metal (which convertes the metal atom to an ion and the ion then leaves the parent metal). If a metal which is more strongly willing to give up elctrons is connected electrically to the parent metal, it will supply the electrons and loose atoms as ions instead. This is the principle behind galvanishing and sacrifical anodes. The sacrificial metal is typically zinc ( but can be aluminium and often is a combination of zinc and aluminium). Hence Zincalume. Galvanising can be very effective. It involves dipping the whole steel structure into a bath of molten zinc. Zincalume coated steel sheets also have an excellent life. Zinc plating immerses the work in a plating bath (usually strongly acid) and passes an electrical current through it.
Sacrificial coatings and anodes are very effective, how-ever they do have a limited life. Eventually they will be fully consumed and then the parent metal will start to corrode instead. There are many items which cannot have these treatments either due to size, or their inability to withstand the mechanical or corrosive cleaning processes, or the heat of the molten zinc or the acidic plating environment. Bolt on anodes require permanent immersion (boats) to be effective.
Physical Barrier Coatings
The idea with physical barriers is to prevent water or oxygen getting to the parent iron or steel. If one or the other (or both) can be excluded, then the standard rust reaction will not occur. Some simple things can give quite good short term protection such as coating the part in grease or oil. This will exclude both moisture and oxygen. There are some extremely effective but also expensive and complicated solutions used by engineers, such as on the Goodwill Bridge in Brisbane, where the insides of the hollow steel members are completely sealed up and filled with dry nitrogen which thus excludes moisture and oxygen from the steel.
Most coating systems fall into this category. A rust control coating to be effective must do a number of things. Firstly, it must be a good barrier to moisture and oxygen. Secodly it must adhere well to the metal being protected. Thirdly it must continue to adhere well and be a good barrier for the required length of time. Fourthly it must be able to withstand the bumps, knocks and scrapes that will occur during its life without the barrier being breached and permitting oxygen and water to reach the protected material. Finally, the coating system should not be seriously toxic, either for the applicator or subsequent perons who come into contact with it and it should not generate intractable waste disposal problems.
All coatings will provide some protection, but in some cases (e.g. water based acrylics) it is very very limited indeed.
Two pack epoxy is probably the coating system which best meets all these needs. Modern ships are often protected by barrier coatings of two pack epoxies. Two pack epoxy is probably the best barrier of all coating polymers available and it has acceptable safety and enivironmental values. It has no inherent degradation mechanism so it has an excellent potential life. It has a couple of limitations. The first is that the steel surface to be protected has to be blast cleaned to a very high degree to ensure that great adhesion longetivity is obtained. The second is that all epoxies are damaged by the ultra violet in sunlight, and it has to be overcoated with other paints which absorb the UV rays.
The classic protection method for steel structures were red lead based primers followed by MIO alkyd undercoats and top coats. This often protected the structure for as much as 50 years, and many steel bridges are still protected by it. The red lead primer is now outlawed and unavailable. To be really effective, the structure needed to be abrasive basted to a high degree of cleanliness prior to applying the primer to ensure the lead could come into contact with the steel and provide an inhibiting effect.
Amongst the single pack coatings, most are OK as barriers. Some of the older ones whilst giving quite good barrier performance have high VOC's ( volatile organic compounds) so they are no longer widely used. Some of the newer ones include single pack epoxies. The professional opinion for instance about single pack epoxy coatings is that they are not very strong and to become effective, they would need a high temperature bake cure. Thus it might be suggested that single pack epoxies are trading on the epoxy name to sound good, possibly without providing great performance. Most require strenuous cleaning of the metal surface to a bright (calss 2.5) surface.
One single pack coating which does offer great barrier performance is a certain vinyl acrylic co-polymer. This has been tested and found to have a very satisfactorily low permeability. It has great adhesion to a wide variety of surfaces. This resin also exhibits a great balance between hardness and flexibility which results in a very tough coating. This in turn means that it will be very resistant to most mechanical damage which could lead to a protection failure. It is water based, non toxic, has minimal irritation issues and its use leads to minimal environmental concerns. This is the base resin which we use in our Feronite Rusty Metal Primer.
Chemical Inhibitors, Conversion Coatings
Inhibitors are chemical substances which react with the metal surfaces so as to prevent the rust reaction continuing. There are both anodic and cathodic inhibitors. With an anodic inhibitor, the inhibitor ion reacts with the metallic ions (Fe+++) formed on the anode surface to form an insoluble hydroxide which is deposited on the metal surface and forms an impermable film. This alters the electrochemistry and effectively passivates the metal in this region. Phosphates are one such anodic inhibitor. Cathodic inhibitors behave similarly at the cathodic area forming a barrier of insoluble precipitates over the cathodic area. This insulates the cathodic area. Zinc for example reacts with the OH- ions to form insoluble Zn(OH)2 zinc hydroxide. Tanins also behave as cathodic inhibitors.
Many coatings (including our Feronite Rusty Metal primer) incorporate ZnPO4 (zinc phosphate) so that if the barrier of the coating is broken through a scratch or bump, the corrosion inhibitor will provide some protection in the damaged area.
Electrical protection
Another approach is to use a small electrical current to reverse the flow of electrons, in effect to over power the tiny battery that is resulting in the steel dissolving. This solution is most frequently applied on underground pipelines and the reinforcing steel inside the concrete on large buildings. It requires that a sacrificial or neutral electrode is located near to the asset being protected and a small voltage (a few volts) with -ve to the item being protected is applied. Close monitoring is required to enure that all parts of the structure are being properly protected. Occasionally, a product appears on the amrket which is said to do the same thing with a car. As there is no way to connect a second electrode and have a current flow between them, there appears to be no scientific basis for these pieces of equipment.
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