Morphology of Corrosion
The corrosion process develops in the metallic materials in several ways:
Stress corrosion cracking
Fig 1 – Uniform corrosion
Fig 2 – Non-uniform corrosion
This type of corrosion involves the whole surface of the metallic material in contact with the corrosive environment. If the attack is spread in a uniform way, it is called uniform corrosion (Figure 1), otherwise it is called non-uniform corrosion (Figure 2). General corrosion produces loss of material through the thinning of the metal with a velocity which is typically predictable if the environmental conditions are known. For example, the corrosion of carbon steel exposed to the atmosphere has a speed that develops from a dozen to several hundreds of microns per year (μm/year) depending on the temperature, humidity, presence of chlorides and or other pollutants. The general corrosion is considered to be the main form of corrosion and, at times, the less destructive than all other forms of corrosion; can occur on carbon steels, stainless steels and zinc in acidic solutions, on carbon steels in environments “sweet” that is, in the presence of carbon dioxide and in the oil&gas manufactory, on aluminium in solutions with lower and higher pH values. The products of corrosion can lead to swelling, reduction of mechanical strength and pollution due to the dissolution of the metal in contact with the corrosive environment (Figure 3).
Figure 3 Locking system corroded
A method to prevent this type of corrosion consists to estimate the “corrosion allowance” (Corrosion Allowance, CA). This parameter defines an increase of thickness of the metal in order to predict the loss of material during the useful life of the part subject to corrosion. Since the depth of penetration can vary from case to case, the risk of corrosion is a factor of safety equivalent to 2. The formula to calculate CA is:
CA=CR · t
Knowing the expected rate of corrosion (CR) expressed in μm/ year and expected duration of mechanical parts (t), the industries are able to calculate the additional thickness needed for the corrosion resistance of the material in the process of design. CA factor must be calculated according to the conditions of use of the metal piece. The parameters that affect the corrosion rate (CR) are:
- Aggressive substances
A very wide knowledge of materials allows to carry out a careful selection of the material in order to optimize the resistance to corrosion. For example, the stainless steel is a material that is very resistant in environments subject to generalized corrosion compared to a carbon steel due to its ability to create a thin protective layer (passivation) which lowers the corrosion rate and increases the useful life of the piece.
Additional measures to reduce or stop the corrosion process are:
- Coating or painting over the metal surface;
- Cathodic protection (metal dip in water or metal in the soil);
- Checking the environment: pH, O2, T, flowing system, stresses, corrosion inhibitors.
Figure 4 Protective coating over the metal surface