November 2010 Archives

Atmospheric corrosion of metals occur by water adhering on surfaces and oxygen in the air. A marked difference of atmospheric corrosion from underwater corrosions is that supply of water is limited in relation to the abundance of oxygen. The water is supplied onto metal surfaces through rainfall and condensations of water vapor in the air.
The condensation is not often visible to naked eyes but is quite responsible for the progress of corrosions. Rainfalls occur outdoors but the indoor corrosions are mostly due to water vapor condensations.
Moisture in the air is normally represented in Relative Humidity term. This is defined as: Percentage of moisture content in the air against a saturated water vapor content as 100% at a "given temperature".

Therefore, there should be no condensation on surfaces unless the relative humidity becomes 100%, theoretically. Condensation would occur if the temperature drops while the relative humidity is at a certain level since the vapor saturation limit would also drop. However, condensations actually occur even if the temperature does not drop for the following two reasons.

　"Erosion" is a wear phenomenon that occurs mechanically, and "Corrosion" is a decomposition phenomenon. The term corrosion represents a "wear due to decomposition".

　As with water flowing in a plumbing pipe, flowing fluid would rub against the inner wall surface. Slow flowing fluid may not cause any wear, but high flow velocity would result in wear. Furthermore, any inclusion of solid particles in the fluid will cause vigorous abrasions and mechanical wear.

　Regarding carbon steels, they are likely to corrode in water and other solutions resulting in rust forming on their surfaces. Unlike the rusts on stainless steels, carbon steel rust is not as corrosion resistant as to prevent any further rusting. But the rust will slow down the speed of further corrosions compared to not having any at all.

　However, if any fluid abrasion effects exist in this state such corrosion byproducts would be scraped away and fresh carbon steel surface would be exposed. If dissolved oxygen in water acts with the fresh surface, new corrosion will progress, naturally. The "Erosion" effect removes the rust and new "Corrosion" effect advances. This compounded effect increases the speed of corrosion, compared to having either one or the other only.

　This can be evidenced by immersing a small piece of carbon steel in salt water solution, and scrape off rust in a specific location 2~3 time per day. In a few months, scraped portion will become indented.

　Actual well known cases of Erosion & Corrosion can be seen on high speed flow water/salts plumbing where turbulences are like to occur. Even on relatively slow flowing plumbing can be subject to this such as with pipe bends and heat exchanger inlets. These locations are likely to be affected by Erosion & Corrosion phenomenon.

　When solids are included in the flowing fluid, such as with coal transporting pipe lines, Erosion & Corrosion is highly like to occur. The phenomenon is also likely to occur with gas-liquid mixtures and gas-mist mixtures.

　Rust resistant stainless steels are also resistant to Erosion & Corrosion phenomenon. The passive surface layer of stainless steel can quickly reform even after being eroded away. However, there are some aggressive Erosion & Corrosion instances of stainless steel where used for salt water pump impellers.

Corrosion Fatigue is a facture phenomenon occurring when repeated stresses are applied on metals in corrosive environments. As with the Stress Corrosion Fracture phenomenon, this does not occur with either stresses or corrosion alone, but both must exist simultaneously.

Let us suppose what metal fatigue is. A wire as an example, will break if repeatedly bent and straightened. This occurs because the metal is fatigued and resulted in a fracture due to stress build-up by repeated bending.

There are many objects that receive the repeated stress in the real world. Railway bridges are subjected to stress every time a train passes, and marine structures such as bridges receive large cyclic stresses when hit by waves. Pressure bulkheads of large aircrafts are subjected to stresses caused by air pressure differentials on take-offs. The metal fatigue occurs in atmosphere, in vacuum, and in water.

Based on investigations into how applied stresses and frequencies affect metal fractures, it is found that metals fracture only with a few bend repetitions if the applied stresses exceed certain limits. This limit is called "Fatigue Limit". When applied stresses are below Fatigue Limits, metals do not fracture even if repeatedly exposed to the stresses. This fact establishes an important indices for designs on machines and equipment that are exposed to repeated stresses.

However, metals can fracture when exposed to repeated stresses at even less than the Fatigue Limit if a corrosive environment is combined. Also, fractures can occur with less repetition cycles at the same stress levels. This means that Fatigue Limit disappears. In other words, metal will fracture, regardless of repeated stress levels, if enough repeated stresses are applied. This is called "Corrosion Fatigue".

In marine structure designing, it is important to design with close attention to metal's corrosion fatigue characteristics. But the metals used are typically all steel alloys, and it is difficult to improve the corrosion fatigue characteristics of steel alloys. For the fatigue factor alone, it suffices to use materials with high fatigue limits, but anti-corrosion property must also be taken in consideration for the corrosion fatigue factor. Although it is not impossible to improve materially, but only within limited ranges. Therefore, the important design approach would be to minimize features and locations where stresses would concentrate, or reduce the amounts of stress concentrations. Marine structures for an example, many small diameter steel tubes are driven into the ground and weld connected to the main support columns, and used as sacrificial corroding member as a countermeasure. Some electrical anti-corrosion measures as explained in a later discussion are in use also.