Corrosion is usually referred to as the degradation of the metal by chemical or electrochemical reaction with its environment, see figure 1. When considered broadly, corrosion may be looked upon as the tendency of the metal to revert to its natural state similar to the oxide from which it was originally melted. Only precious metals, such as gold and platinum are found in nature in their metallic state.
Some metals produce a tight protective oxide layer on the surface, which hinders further corrosion. If the surface layer is broken it is self-healing. These metals are passivated. Under atmospheric conditions the corrosion products of zinc and Aluminium form a fairly tight layer and further corrosion is prevented.
Likewise, on the surface of stainless steel a tight layer of iron and chromium oxide is formed and on the surface of titanium a layer of titanium oxide is formed. The protective layer of these metals explains their good corrosion resistance. Rust, on the other hand, is a non-protective corrosion product on steel. Rust is porous, not firmly adherent and does not prevent continued corrosion, see figure 2.
Types of corrosion
Generally, metallic corrosion involves the loss of metal at a spot on an exposed surface. Corrosion occurs in various forms ranging from uniform attacks over the entire surface to severe local attacks.
The environment’s chemical and physical conditions determine both the type and the rate of corrosion attacks. The conditions also determine the type of corrosion products that are formed and the control measures that need to be taken. In many cases, it is impossible or rather expensive to completely stop the corrosion process; however, it is usually possible to control the process to acceptable levels.
On the following pages we will go through the different forms of corrosion in order to give you an idea of their characteristics.
Uniform or general corrosion is characterized by corrosive attacks proceeding evenly over the entire surface, or on a large part of the total area. General thinning continues until the metal is broken down. Uniform corrosion is the type of corrosion where the largest amount of metal is wasted.
Examples of metals, which are subject to uniform corrosion:
- Steel in aerated water
- Stainless steel in reducing acids (such as EN 1.4301 (AISI 304) in sulfuric acid)
Pitting corrosion is a localized form of corrosive attacks. Pitting corrosion forms holes or pits on the metal surface. It perforates the metal while the total corrosion, measured by weight loss, might be rather minimal. The rate of penetration may be 10 to 100 times that of general corrosion depending on the aggressiveness of the liquid. Pitting occurs more easily in a stagnant environment.
Example of metal that is subject to pitting corrosion:
- Stainless steel in seawater
Crevice corrosion – like pitting corrosion – is a localised form of corrosion attack. However, crevice corrosion starts more easily than pitting. Crevice corrosion occurs at narrow openings or spaces between two metal surfaces or between metals and non-metal surfaces and is usually associated with a stagnate condition in the crevice. Crevices, such as those found at flange joints or at threaded connections, are thus often the most critical spots for corrosion.
Example of metal that is subject to crevice corrosion:
- Stainless steel in seawater
As the name implies, intergranular corrosion occurs at grain boundaries. Intergranular corrosion is also called intercrystallite corrosion. Typically, this type of corrosion occurs when chromium carbide precipitates at the grain boundaries during the welding process or in connection with insufficient heat treatment. A narrow region around the grain boundary may therefore deplete in chromium and become less corrosion resistant than the rest of the material. This is unfortunate because chromium plays an important role in corrosion resistance.
Examples of metals that are subject to intergranular corrosion:
- Stainless steel – which is insufficiently welded or heat-treated
- Stainless steel EN 1.4401 (AISI 316) in concentrated nitric acid
Selective corrosion is a type of corrosion which attacks one single element of an alloy and dissolves the element in the alloy structure. Consequently, the alloy’s structure is weakened.
Examples of selective corrosion:
- The dezincification of unstabilised brass, whereby a weakened, porous copper structure is produced
- Graphitisation of gray cast iron, whereby a brittle graphite skeleton is left because of the dissolution of iron
Erosion corrosion is a process that involves corrosion and erosion. The rate of corrosion attack is accelerated by the relative motion of a corrosive liquid and a metal surface. The attack is localized in areas with high velocity or turbulent flow. Erosion corrosion attacks are characterized by grooves with directional pattern.
Examples of metals which are subject to erosion corrosion:
- Bronze in seawater
- Copper in water
A pumped liquid with high velocity reduces the pressure. When the pressure drops below the liquid vapor pressure, vapor bubbles form (the liquid boils). In the areas where the vapor bubbles form, the liquid is boiling. When the pressure raises again, the vapor bubbles collapse and produce intensive shockwaves. Consequently, the collapse of the vapor bubbles remove metal or oxide from the surface.
Examples of metals that are subject to cavitation:
- Cast iron in water at high temperature
- Bronze in seawater
Stress corrosion cracking (SCC)
Stress corrosion cracking (SCC) refers to the combined influence of tensile stress (applied or internal) and corrosive environment. The material can crack without any significant deformation or obvious deterioration of the material. Often, pitting corrosion is associated with the stress corrosion cracking phenomena.
Examples of metals that are subject to stress corrosion cracking:
- Stainless steel EN 1.4401 (AISI 316) in chlorides
- Brass in ammonia
Pure mechanical fatigue is when a material subjected to a cyclic load far below the ultimate tensile strength can fail. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at an even lower stress and after a shorter time. Contrary to a pure mechanical fatigue, there is no fatigue limit in corrosion-assisted fatigue.
Example of metals that are subject to corrosion fatigue:
- Aluminium structures in corrosive atmosphere
When a corrosive electrolyte and two metallic materials are in contact (galvanic cell), corrosion increases on the least noble material (the anode) and decreases on the noblest (the cathode). The increase in corrosion is called galvanic corrosion. The tendency of a metal or an alloy to corrode in a galvanic cell is determined by its position in the galvanic series. The galvanic series indicates the relative nobility of different metals and alloys in a given environment (e.g. seawater).
The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect will be. Metals or alloys at the upper end are noble, while those at the lower end are least noble.
Examples of metal that are subject to galvanic corrosion:
- Steel in contact with 1.4401
- Aluminium in contact with copper
The principles of galvanic corrosion are used in cathodic protection. Cathodic protection is a means of reducing or preventing the corrosion of a metal surface by the use of sacrificial anodes (zinc or aluminum) or impressed currents.