Corrosion is one of the leading causes of failure. Due to the vast number of materials that are affected by corrosion, different types of corrosion, and different ways to address the causes of corrosion, finding the correct solution requires a fundamental understanding of the underlying causes and mechanisms.
What is corrosion?
The corrosion of metal can be defined as a chemical or electrochemical reaction between the surface of the metal and its environment. In aqueous environments corrosion is almost always electrochemical in nature. It occurs when two or more electrochemical reactions take place on a metal surface, cauUsing some of the elements of the metal or alloy to change from a metallic state into a non-metallic state in order to decrease the energy of the system. The products of corrosion may be dissolved species or solid corrosion products.
The corroding system comprises anodic and cathodic sites, which are electrically in contact, and a surrounding electrolyte. According to mixed-potential theory, i) the electrochemical reaction can be algebraically divided into two (or more) oxidation and reduction half-reactions, and ii) there can be no net electrical charge accumulation. When metal corrodes in an aqueous solution, the metal anode is oxidised (loses electrons) to form metal ions (typically M2++), releasing electrons in the process. These electrons are consumed in the cathodic reaction, which is dependent on the nature of the electrolyte. In acid solutions the hydrogen cations in the acidic solution are reduced (gain electrons) to form atomic hydrogen, which subsequently combines to form hydrogen gas – a process called ‘hydrogen evolution’. In neutral or basic solutions, such as sea water (which typically has a pH of 7.5 – 8.3), oxygenated water is reduced to hydroxyl (OH–) ions.
In sea water, steel is transformed into metal oxides or hydroxides. For example, in the case of steel submerged in sea water, ferrous hydroxide precipitates from solution, but is unstable in oxygenated water and is further oxidised to form ferric salt, Fe(OH)3, which is the all too familiar ‘rust’.
Why does corrosion occur?
In addition to the mechanism describe in the preceding section, corrosion is often driven by other mechanisms that include:
- Galvanic corrosion cells are readily formed between two dissimilar metals that are in electrical contact and exposed to a common electrolyte (e.g. seawater). Provided the galvanic potential of the materials (i.e. of the anode and cathode sites) differs by more than approximately 250mV, the more active metal (the anode) will be oxidised to form metal ions (typically Mz++), releasing electrons in the process. These electrons flow through the electrical path between the materials where they are used in the cathodic half-reaction, which in the case of neutral or alkaline environments such as oxygenated seawater, is the reduction of oxygen to form hydroxyl ions (OH–). Galvanic corrosion causes significant degradation of the anodic material.
- Crevice corrosion occurs most commonly in interfaces between metals and other components where the flow of electrolyte is limited. Due to the limited flow, corrosion of the exposed surfaces in the crevice reduces the amount of oxygen in the electrolyte within the crevice and the reduction reaction slows. As the oxidation of metal continues a positive charge develops within the crevice, which accelerates the diffusion of ions such as chlorides and sulphates into the crevice. These ions combine with the metal ions in the crevice to form metal salts which in turn hydrolyse to from an insoluble hydroxide and a free acid. Hydrogen, chlorides and the free acid all increase the rate of metal dissolution in an autocatalytic reaction.
- Pitting corrosion is highly localised corrosion and is usually caused by localised metal dissolution, or a breakdown of passivity resulting in a small, electrolytic cell. The corrosion pit tends to extend (often vertically into the metal under the action of gravity) as the material in the pit oxidises, forming metal ions (MZ+), and supplying electrons to the surrounding cathodic area. The rapid dissolution of the metal within the pit tends to cause a build-up of charge within the pit which, as in the case of crevice corrosion, tends to attract chloride and sulphate ions into the pit, which combine with the metal ions to form metal salts. Subsequent hydrolysis of the metal salts causes an increase in concentration of hydrogen and chloride ions which increase the rate of metal dissolution within the pit. Due to the low solubility of oxygen in the concentrated solution within the pit, very little oxygen reduction occurs within the pit itself but rather occurs in areas adjacent to the pit which tends to suppress the corrosion of the material surrounding the pit.
- MIC or Microbial Induced Corrosion refers to the corrosion damage of metal surfaces caused by localised depolarization and changes to the local environment by bacteria/micro-organisms, resulting in the onset of pitting attacks on the surface.
- Flow induced/accelerated corrosion takes place when the protective oxide surface layer of a material is depleted by the constant flow of a liquid. The exposed surface then oxidises to recreate the layer.
How do we establish the cause of corrosion?
Depending on the type of corrosion and nature of the components in question, methods to determine the cause(s) include:
- Visual inspection and documentation which involves the visual inspection of the components and assessment and subsequent recording of the operating conditions by suitably qualified and experienced corrosion engineers.
- Destructive analysis of material involves sectioning of the material; microstructural inspection (light and (if required) scanning electron microscopy SEM); and chemical analysis of electrolyte, material and corrosion products.
- Potential surveys are carried out by undertaking measurements of the potential of the structure with respect to a suitable reference electrode. The technique is very useful when assessing the efficacy of the cathodic protection systems employed (e.g. ships, piers, pipelines, tanks…).
- Immersion testing is conducted by immersing a specimen in an electrolytic solution and monitoring material changes.
- Electrochemical testing is typically conducted by either applying an external current (as control with a galvanometer) to generate an electrochemical response/potential, or by taking measurements at a freely corroding potential, without the use of an external current.
- Model testing involves using a scale model of relevant components or structures in a controlled environment to determine the extent, and rate, of corrosion.
- Review/analysis of existing corrosion protection data can often reveal under what circumstances the corrosion accelerated or slowed in the past. This can give some indication as to the cause.
How can we protect against corrosion?
- Isolation – In situations where dissimilar metals are in contact in a conducting aqueous environment, galvanic corrosion can readily occur. Breaking electrical contact between the metals (isolating them from each other and from the electrolyte) should in theory prevent corrosion from occurring. However, this method of mitigation relies heavily on ensuring that there is no contact between dissimilar components, and any break in isolation will result in corrosion occurring (and probably at an accelerated rate – especially in areas adjacent to the interface between the two dissimilar materials).
- Protective coatings – Coatings form a physical barrier between the metal and the corrosive environment/electrolyte. As protective coatings are permeable to some extent and are never defect free, coatings are not 100% effective at stopping corrosion, and are often used in conjunction with another corrosion protection method such as cathodic protection.
- Cathodic Protection
- Sacrificial Anode Cathodic Protection (SACP) – makes use of sacrificial anodes connected to the underlying material by metallic conductors. As these sacrificial anodes are chosen to be more electronegative than the material to be protected, they corrode sacrificially. As they corrode they supply electrons to the material being protected and reduce the potential of the material being protected. Effectively the anode makes the material being protected more cathodic reducing the likelihood of it corroding (more electronegative when their potential is measured relative to a reference electrode).
- Impressed Current Cathodic Protection (ICCP) – works similarly to SACP, but instead of the electrons being provided by the corrosion of a passive sacrificial material, a transformer rectifier unit is used to supply electrons to the structure (structure connected to the negative output of the transformer rectifier unit). This forces the structure to become cathodic with respect to an inert anode that is connected to the positive terminal of the transformer rectifier system and placed in the electrolyte (but is isolated from the structure being protected).
- Sacrificial coatings – coating components with thin layers of metal, with a more negative potential than that of the material being protected, can be used to protect the underlying material. The electrons supplied during the sacrificial oxidisation of the protective coating protect the underlying material and in reality can be considered as a form of SACP.
- Corrosion inhibitors – are chemical compounds which attach or react with a material, creating a thin, protective film. Some of these protective films enhance the natural oxide layer of the material to prevent corrosion, others aim to prevent ion flow from the underlying material.