December 2010 Archives

A variety of metallic objects such as gas, water, communication, and power lines are ground embedded. Especially in recent years the oil and natural gas lines are used for large scale long distance distribution of energy sources, and corrosion protection measures for such infrastructure has become of important safety concerns. The corrosions occurring in soil differ slightly from those occurring in atmosphere and water. Soil is comprised of mixtures of soil particles, water, and air. The corrosions in the soil have some similarities with those occurring in air with thin water layer, but different where oxygen supply is much less and wet-dry cycles are less frequent. Additionally, the ground water contains various dissolved elements and have varying water retention characteristics/humidity, causing a wide variety of corrosions due to varying oxygen supplies, diffusion rates, and water/oxygen reactions.

Factors influencing the soil corrosions are pH, electrical conductivity, water content percentage, drainage characteristics, air permeability, and chemical composition (ion types and concentration levels). The electrical conductivity is related to moisture content and ion concentration, water content percentage and drainage characteristics are related to moisture content. Porosity of soil limits the oxygen and water supplies and corrosions of steel in soil would be less compared to being in air or water. If corrosion were to progress uniformly, it is said to be 0.06mm/year at the most.
However, soil is an environment that makes steel corrosions quite non-uniform. The important factor is the amount of oxygen diffusion onto metal surfaces, and it varies depending on porosity and water content of the soil. Corrosions progress more with more oxygen, but at times oxygen concentration differences create concentration differential cells that accelerates corrosions.
An example of soil composition related problem would be a case of soil mixture of sand and clay. If plumbing is to be embedded in such a soil mixture, localization of corrosions would occur due to higher air permeability of sand.

A phenomenon of metal corrosions occurring when DC currents flow from metals to environments were previously discussed. This can also occur even without corrosion battery mechanism. Currents leaking from DC electric trains and metal smelting plants to the soil would enter embedded structures such as pipelines and again exit to the soil. These are called "Stray Currents" and corrosions caused by these currents in the soil are often seen as problems. Similar corrosions can also be seen in fresh and sea water, though not as often as in the soil, and are called "Stray Current Corrosions". Various countermeasures are being considered, but the applications are limited to coating with coal-tar type paints and electrolytic Corrosion protection methods due to cost limitations.

When steel is immersed in water with a small amount of salt, significantly accelerated corrosion will occur. Natural sea water salt concentration is said to be an average of 3% (30g/L), though there are variations based on locations, and is the most corrosive of all natural environments. The corrosiveness increases as the water flow rate increases. The actual corrosions vary depending on locations/seasonally since the natural sea water is quite complex in composition containing other corrosive components other than salts such as dissolved oxygen.

When examining the corrosion of steel by sea water, the rate of corrosion is far more severe when steel is repeatedly immersed and pulled out, compared to just being left immersed in water. For instance, consider steel pilings used as shore protection for ports.
Steel pilings are driven into the ocean floor along the shore-line, and contact the ocean floor mud extending through the sea water to the water line. The most severe corrosion occurs at just above the water line where the waves splash. This area is called the "Splash Zone". In the water, dissolved oxygen amount is limited and corrosion progress would be limited, but abundance of oxygen is supplied from the atmosphere in the Splash Zone and the corrosion progresses rapidly. The corrosion progresses more rapidly out of the water, compared to being immersed under the water, since the salt deposits on the metal surface attracts humidity coupled with abundance of oxygen supply through thin layer of water remaining on the metal surface.
The fact that corrosions decelerate after the metal is pulled out of fresh water but accelerates with sea water, means the corrosions are promoted when metals are wetted with sea water in the atmosphere.

For other marine structures not limited to the steel pilings such as offshore airports, connecting bridges, marine city structures, and offshore oil rigs are subject to Corrosion protection countermeasures. The countermeasures are classified based on applicable corrosive environmental factors such as air, plash zones, tidal zone, sea water, ocean floor soil, etc.
For the most severe corrosion environment of splash zones, wrapping of corrosion resistant alloys such as 2~3mm thick Monel (70% Ni, 30% Cu) or pilings made of Monel clad metal are used, since normal paint with 200~300 Micrometer thickness film would be ineffective.
Recently, organic linings as thick as 10mm of resin mortar, tar epoxy, and urethane rubber are used effectively.
For corrosion prevention in electrolyte (sodium chloride, etc.) rich sea water and marine soil environments, electrical corrosion protection measures are in use. (Electrical corrosion protection measures are discussed later)

Steel corrosion progress by water depends on the amount of oxygen dissolved in the water. The dissolved oxygen concentration supplied by atmosphere at normal temperatures is very low of 8~10ppm, so the speed of corrosion progress will depend on dissolved oxygen diffused from water.

If no foreign objects are adhering on the steel surface, the speed of steel corrosion in static fresh water is 0.4mm/year based on dissolved oxygen concentration and diffusion rate. In actuality, it is said that formation of rust would slow down the rate to 0.1mm/year.

Not as common in Japan where surface water flow are rapid due to the country's mountainous terrain, but in Europe and the Americas where the lands are flatter and the surface water flows slowly the water hardness can be high containing much calcium from the ground and rocks. When steel comes in contact with hard water, a surface layer of calcium carbonate is formed inhibiting the dissolved oxygen, and significantly delaying the corrosion process.

It is said that water influences on corrosion in Japan is relatively insignificant within pH of 5~9, and concentrations of chloride and sulfate ions are also insignificant in static water condition. This is indicative of the fact that dissolved oxygen concentration level determines the rate of corrosions.

When there is a significant fluid flow, the amount of dissolved oxygen supply would increase and the corrosion rate would also increase as the flow rate increases. If the flow increases beyond certain rates, dissolved oxygen supply becomes excessive and cannot all be consumed for steel dissolution, and passive layer formation would begin, then the corrosion rate would decrease. This also occurs when the water temperature decreases and dissolved oxygen concentration increases.

The effects of chloride and sulfate ions in fresh water on steel corrosions, regarding the fluid flow conditions for passive surface layer formation and post passive layer formation corrosion rates, become larger as ion concentrations increase. In water high in chloride ion concentration such as sea water, passive layer formation does not occur even if the flow rate is increased. Generally, chloride ions are known to be a corrosion promoting agent, but in the case of water immersed carbon steels the negative effects apply only when the flow rate is at over Critical Flow.

Steel corrosion rate in water increases up to 80°C, but begins to decrease when that is exceeded. This is due to a conflicting effect on the corrosion rate of dissolved oxygen diffusion rate increase by the increased temperature, while oxygen solubility decreases.

Stainless steels and aluminum do not corrode in fresh water, in principle, but Pitting Corrosions and Crevice Corrosions will become of issues when chloride ions exist in the water.