The following will treat corrosion as a process which cannot occur without the presence of water and therefore excludes other types of attack, such as those associated with high temperature oxidation or sulphidation.
Corrosion is an electrochemical process in which a metal reacts with its environment to form an oxide or other compound. The cell which causes this process has three essential constituents: an anode, a cathode and an electrolyte (electrically conducting solution). The anode is the site at which the metal is corroded; the electrolyte is the corrosive medium; and the cathode (part of the same metal surface, or of another metal surface in contact with it) forms the other electrode in the cell and is not consumed in the corrosion process. At the anode the corroding metal passes into the electrolyte as positively charged ions, releasing electrons electrons which participate in the cathodic reaction. Hence the corrosion current between anode and the cathode consists of electrons flowing within the metal and ions flowing within the electrolyte.
The surface of one component may become the anode and the surface of another component in contact with it the cathode. Usually, corrosion cells will be much smaller and more numerous, occurring at different points on the surface of the same component. Anodes and cathodes may arise from differences in the constituent phases of the metal itself, from variations in surface deposits or coatings on the metal, or from variations in the electrolyte.
The metal may be immersed in an electrolyte or the electrolyte may be present only as a thin condensed or adsorbed film on the metal surface. The rate of corrosion is influenced considerably by the electrical conductivity of the electrolyte. Pure water has poor electrical conductivity and the corrosion rate will be much lower than say an acid solution of high conductivity.
The ability of metals to resist corrosion is to some extent dependent upon their position in the electrochemical series.
Electrode Potential (Volts)
Hydrogen Overvoltage (Volts)
-1.87 (Base End)
+1.3 (Noble End)
The farther two metals are separated from one another in the electrochemical series, the more powerful is the electric current produced by their contact in the presence of an electrolyte. Also the more rapidly the metal towards the top of the table is attacked and the more will the metal towards the bottom of the table be protected. It must be remembered, however, that the order in the above series may vary under special corrosive conditions, and the galvanic series in service media, e.g. sea water, are often more useful from the corrosion aspect.
GALVANIC SERIES IN SEA WATER
CORRODED END (Anodic)
PROTECTED END (Cathodic)
An example of a corrosion cell is provided by an imperfect coating of copper on steel immersed in dilute sulphuric acid. The current generated passes from the copper to the steel by the path of lowest resistance and returns to the copper through the solution by the passage of ions. The steel, which has the greatest negative potential, dissolves and is called the anode; whilst the copper is called the cathode. In such acid attack the hydrogen, which is freed as the iron dissolves, is deposited on the surface of the copper cathode and as it increases in amount two things may occur. The corrosion of the steel is either reduced because of the formation of an opposing hydrogen electrode, i.e. the cell is polarised; or the hydrogen may be evolved as bubbles which stream away, with the result that the corrosion will occur continuously. In the first case the corrosion will be accelerated by exposure to oxidising agents (e.g. air) which remove the hydrogen from the cathode. The size of the cathode relative to the anode is important, e.g. copper rivet in a large steel plate, is quickly polarised and corrosion on the plate is small. On the other hand, a large cathode coupled to a small anode has the opposite effect, with rapid attack of the anode.
Iron and steel are the most common materials of construction, and their corrosion characteristics in neutral waters is important. When steel corrodes, the corrosion rate is usually governed by the cathodic reaction of the corrosion process, and oxygen is an important factor. In neutral waters free from dissolved oxygen, corrosion is usually negligible. The presence of dissolve oxygen in the water accelerates the cathodic reaction; and consequently the corrosion rate increases in proportion to the amount of oxygen available for diffusion to the cathode. Where oxygen diffusion is the controlling factor, the corrosion rate tends to increase also with rise in temperature. In acid waters (pH <4), corrosion can occur even without the presence of oxygen.
Corrosion of steel under a droplet of water
Pitting and crevice corrosion
Electrochemical corrosion can be stimulated from not only differences in the metal surface, but also from variations in the electrolyte. The above is effected to some degree by this mechanism, as oxygen diffuses into the water drop a concentration gradient is set up, where the oxygen content at the extremities is the highest and the lowest being at the centre where the anode forms. Cavities in metal surfaces and metal surfaces partially covered by another material are prone to this type of attack. The diffusion of oxygen into cavities or crevices is impeded and results in these areas becoming anodic to the surrounding metal to which oxygen can easily reach (oxidation-concentration cell or differential aeration cell). The metal ions formed in the cavity migrate outwards and react with the hydroxide ions flowing in the opposite direction to form a corrosion product (rust) at the mouth of the cavity or crevice. This position of the corrosion product accentuates the corrosion by making the diffusion of oxygen to the anode more difficult, and if the cathodic area is large severe pitting may occur. Also when dry conditions prevail moisture can be trapped in the cavities allowing corrosion to continue.
Normally very corrosion resistant materials which rely on thin oxide films for protection, such as stainless steel, can suffer from this type of corrosion attack. These materials rely on oxygen being present, so that they can maintain their oxide films (passive state). When oxygen is excluded and the oxide films break down, the material surface becomes active and corrodes readily.(See galvanic series)
The effects of corrosion can be accelerated or induced when operating in conjunction with stress and various wear mechanisms. Usually the mechanisms work by not allowing the corroded metal to become passive by continually removing protective films and setting up active/passive corrosion cells where the mechanism is not uniform applied. The corrosion products formed may provide abrasive debris to make matters worse.
Ideally, a material which is inherently resistant to its service environment, meets with the mechanical, formability and economic requirements would be the first choice for selection. Unfortunately, this is not often the case. Many materials will need a method of corrosion control and there are three main approaches:
Thermal spray coatings are widely used in preventing corrosion of many materials, with very often, additional benefits of properties such as wear resistance etc. due to the very wide selection of coatings that can be sprayed. Broadly, thermal spray coatings fall into three main groups:
Anodic coatings for the protection of iron and steel substrates are almost entirely limited to zinc and aluminium coatings or their alloys. Where coatings anodic to the substrate are applied, the corrosion protection is referred to as cathodic protection or sacrificial protection. The substrate is made to be the cathode and the coating the sacrificial corroding anode. The mechanisms of corrosion protection inferred by these coatings fall into two classes:
An ordinary sprayed coating of zinc or aluminium although somewhat porous, to a large extent excludes the environment and provides cathodic protection. Where desired the porosity can be sealed with organic sealers, or the coating painted, which can in some cases prolong the life of the protective system by increasing the barrier effect. It is argued that sealing or painting these coatings reduce the cathodic protection effect and reduce the overall effectiveness. Sealing and painting certainly reduces the cathodic protection by decreasing the area of contact of the coating with the environment, but in many cases where the substrate becomes exposed there is more than enough coating exposed to keep the substrate as the cathode. Also, the barrier effect prolongs the life of the coating. It could be argued that this is not applicable to all situations. Depending on choice of coating system and environment the life expectancy can be well in excess of 20 years with no maintenance. This method is generally regarded as providing superior corrosion protection than galvanising, plating and painting without excessive cost penalties.
Cathodic coatings are those which comprise a coating metal which is cathodic with respect to the substrate. A stainless steel or nickel alloy coating would be cathodic to a steel base. Cathodic coatings can provide excellent corrosion protection. There is a very wide choice particularly for steel base materials ranging from stainless steel to more exotic materials like tantalum to cater for the more extreme corrosive environments. However, an outstanding limitation of such coatings is that they must provide a complete barrier to the substrate from the environment. If the substrate is exposed to the corrosive environment, the substrate will become the anode and corrosion will be dramatically accelerated resulting in spalling of the coating.
Generally, sealing of these coatings is always recommended. Processes which provide the densest coatings are preferred (HVOF, plasma and fused coatings). Thick coatings will provide better protection than thin coatings.
Neutral materials such as alumina or chromium oxide ceramics provide excellent corrosion resistance to most corrosive environments by exclusion of the environment from the substrate. Generally a neutral material will not accelerate the corrosion of the substrate even if the coating is somewhat permeable (An exception to this is with stainless steel type materials where the exclusion of oxygen can cause crevice corrosion, nickel chromium bond coats are required to stop this), although any corrosion of the substrate interface with the coating should be avoided to prevent coating separation. Again sealing of the coatings is recommended. The densest and thickest plasma sprayed coatings are recommended.