February 2011 Archives

Painting has been in long use for isolating metal surfaces from various corrosive environments. Painting here means to coat metal surfaces with paint and create layers to protect the metal surfaces from corrosive environments.
Painting is applied to virtually everything from ground structures such as bridges, buildings, towers, chemical plants, and marine structures such as shore protections, off-shore structures, as well as various vehicles and home appliances. It is said that 60% of all corrosion protection costs are spent on painting.

Paints are comprised of mixtures of insoluble pigment particles suspended in "vehicle" components that form the coating layers. Generally, solvents are added to the former to give fluidity. This is applied on metal surfaces and let naturally or heat evaporate the solvents for the coating layer to form by chemical reactions.
Natural oils such as linseed and tung oils are customarily used as the "vehicle'. These belong in the drying oil category where oxidation and polymerization occur and harden when exposed to air.
The pigments added to the vehicle are: titanium oxide for white, carbon black for black, and Prussian blue for blue coloring. For the adjustments of glossiness, strength, and volume of the coatings, barium sulfate, calcium carbonate, clay, and diatomaceous earth are used.

The most important Corrosion protect element is the Corrosion protect pigment. Lead tetroxide Pb₃O₄ and zinc chromate ZnCrO₄ are historically being used. They function as corrosion inhibitors by dissolving into water that passes through the coating layer.
The important trait for these pigments would be the dissolvability in water. If the dissolvability is insufficiently low the corrosion protection would be ineffective, and if the dissolvability is excessively high the premature runoff would occur.
Oil based corrosion protection paints have been in wide use. These are comprised of lead tetroxide added to natural vehicles such as linseed oil, and were commonly used due to good economy offered. But recent advancements in high polymerization technology resulted in more use of excellent synthetic resin based paints. The current mainstream choice are phthalic acid, chlorinated rubber, urethane, epoxy, and vinyl chloride resins.

Paint coating method is the easiest and applicable to virtually all shapes and sizes and relatively lower in cost compared other methods. However, the Corrosion protection effectiveness can vary largely depending on the coating application techniques used. This means that it is relatively difficult to always maintain the quality level at a constant.
The quality variations may come from application specific coating layer formation processes, base metal preparation condition where the surfaces are more prone to result in rusts as well as some conditions that may prohibit the coating from adhering perfectly. The pre-coating preparation largely affects the final outcome.

(3) Anodic Corrosion protection

This is a method of corrosion protection by passivation of metals using anode polarization. It was initially adopted for stainless steels which require relatively small electrical potential for passivating and maintaining the passivated states. Thereafter, it was used for Ti and Ni, then for steels today.

When applying the anodic corrosion protect methods, following three points would be the issues

(1) Required Max. Electrical Curren

If the peak currents required to passivate active state metals are large, high capacity power supplies would initially be needed, and this would be uneconomical. However, small currents would be enough to slowly passivate as opposed to the large currents needed for rapid passivation.

(2) Passivation Electrical Potential Ranges

If the metal's passivation potential range is narrow, the anodic corrosion protect method would be difficult to apply since the metal may be re-activated or excessively passivated. The range is considered to be at least 50mV.

(3) Passivated State Maintenance Current

Certain current values are needed to maintain the metals in passivated states. Required maintenance current value increases with temperature increase as well as corrosiveness of the environments.

[Table 1] Passivation Maintenance Current Value

（3）Comparison of Sacrificial Anode Method and External Power Supply Method

The advantages of the External Power Supply Method are: (1) voltage and current are easily adjusted, (2) therefore, is adoptable to changing corrosive conditions, (3) semi-permanent installation is possible if the anodes are sufficiently dissolvable, therefore, is economical.
The Sacrificial Anode Methods, on the other hand, are (1) installations are simple and require no maintenance for certain periods of time, (2) installable where no power source is available or on small equipment, and economical. The shortcomings of the External Power Supply Method are: (1) requires cumbersome current adjustments (costs are high for automatic adjustment systems), (2) the anodes may become inoperative if not sufficiently strong or dissolvable, (3) high initial costs, etc
The shortcomings of the Sacrificial Anode Methods are: (1) Corrosion protect effects may not be sufficient if anode placements in environments are improper, (2) long term corrosion protection cannot be expected without replacing the anodes, typically requiring replacements in a few years intervals.

(2) Problems of Anodic Corrosion protect Methods

There are following problems associated with Anodic Corrosion protect methods.

(1) Electrical current required for corrosion protection

The electrical current required for corrosion protection varies by corrosive environments. For instance, electrical resistance of fresh water is 100 times or more higher than that of sea water. The current required for corrosion resistance depends on the amount of diffused oxygen and does not vary whether fresh or sea water.
Therefore, the electrical power required is the same but the voltage requirement is much higher due to high electrical resistance of the fresh water (same for soil).

(2) Excessive Corrosion protect effects

Excessive cathode polarization results in waste of electrical power as well as many other problems. The hydrogen generated by cathode polarization may cause hydrogen embrittlement fractures on metals that are prone to occlude hydrogen. Additionally, amphoteric metals corrode due to raised cathode pH values.

(3) Corrosion protect measures for inside of pipes

For cases where electrical current flow and corrosion protection distance may be limited such as insides of pipes, corresponding electrode placements are required. Any precipitated deposits of cathode polarization would accelerate the corrosions.

(4) Impingements on adjacent metals

When an electrical current is applied to a target object, electrical corrosion may occur on adjacent metals. This is more of an issue for high electrical resistance environments such as in the ground than sea water. Since it is often not clear what embedded objects are near by in the ground, the installation must follow a thorough investigation.