Corrosion

Ash tray

Steel roof

Hydrogen evolution

Brass plate

White rust on seaside road railing Zn + H2O +O2 p Zn(OH)2 Zn(OH)2 p ZnO + H2O

zinc is exposed to water or moist air followed by natural drying. The resulting zinc oxide is the whitish deposit seen on galvanized pails, rain gutters, and imperfectly chromeplated bathroom faucets. It also familiarly called 'white rust' a non-protective and even destructive form of corrosion that attacks incompletely passivated galvanized steel material or galvanized components subjected to marine atmospheres.

Corrosion Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. Physicochemical interaction between a metal and its environment which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part. Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others.

Corrosion Corrosion is the primary means by which metals deteriorate. It specifically refers to any process involving the deterioration or degradation of metal components. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion processes are usually electrochemical in nature, having the essential features of a battery.

Why Metals Corrode? The thermodynamic or chemical energy stored in a metal or that is freed by its corrosion varies from metal to metal. It is relatively high for metals such as magnesium, aluminum, and iron. It is relatively low for metals such as copper, silver and gold.

Cycle for iron The most common iron ore, hematite, is an oxide of iron. The most common product of the corrosion of iron, rust, has a similar chemical composition and colour. The energy required to convert iron ore to metallic iron is returned when the iron corrodes to form the original compound. Only the rate of energy change may be different.

Pilling-Bedworth ratio The Pilling-Bedworth ratio (P-B ratio), in corrosion of metals, is the ratio of the volume of the elementary cell of a metal oxide to the volume of the elementary cell of the corresponding metal (from which the oxide is created). On the basis the P-B ratio, it can be judged if the metal is likely to passivate in dry air by creation of a protective oxide layer. Definition RPB = Voxide/Vmetal = Moxide.V metal/n.Mmetal. V metal The P-B ratio is defined as: Where:RPB is the Pilling-Bedworth ratio,M - the atomic or molecular mass,n - number of atoms of metal per one molecule of the oxide - density, and V - the molar volume.

Application On the basis of measurements, the following connection can be shown: RPB < 1: the oxide coating is broken, no protective effect (for example magnesium) RPB > 2: the oxide coating chips off, no protective effect (example iron) RPB = 1 2: the oxide coating is passivating (examples aluminum, titanium, chromium-containing steels). However, the exceptions to the above P-B ratio rules are numerous. Many of the exceptions can be attributed to the mechanism of the oxide growth: the underlying assumption in the P-B ratio is that oxygen needs to diffuse through the oxide layer to the metal surface; in reality, it is often the metal ion that diffuses to the air-oxide interface.

Evolution of hydrogen

Electrochemical reactions occurring during the corrosion of magnesium in neutral water (Absorption of oxygen)

Electrochemical reactions occurring during the corrosion of zinc in aerated hydrochloric acid

Pitting corrosion Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against.

Corrosion pit shapePitting corrosion can produce pits with their mouth open (uncovered) or covered with a semipermeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. In some cases they are flat-walled, revealing the crystal structure of the metal, or they may have a completely irregular shape. Pitting corrosion occurs when discrete areas of a

material undergo rapid attack while most of the adjacent surface remains virtually unaffected.

Trough Pits Narrow, deep

Shallow, wide

Eliptical

Sideway Pits Subsurface

Undercutting

Horizontal grain attack

Pitting is initiated by Localized chemical or mechanical damage to the protective oxide film. Water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater) Localized damage to, or poor application of, a protective coating The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions.

Intergranular corrosion The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) Specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grainboundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries. In any case the mechanical properties of the structure will be seriously affected.

Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum (picture width = 500 mm)

Stress corrosion/environmental cracking It is the cracking induced from the combined influence of tensile stress and a corrosive environment. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses Residual stress = Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses

Intergranular SCC of an heat exchanger tube with the crack following the grain boundaries

Stress corrosion

"Season Cracking/environment corrosion "Season Cracking" was the first term used to describe the environmental cracking phenomenon of brass cartridges stored in stables during the monsoon season. In the late 19th century, British rule in India was at its strongest. During the wettest months of the monsoon season, military activity was diminished and ammunition was stored in the stables until dry weather returned. Many cartridges were found cracked. It was not until 1921 that the "season cracking" phenomenon was satisfactorily explained by Moor, Beckinsale and Mallisinson. Ammonia from horse urine, combined with the residual stress, caused the cracking of brass cartridges. This term was later replaced by "stress corrosion cracking" which in turn was replaced by a even more general term of "environmental cracking".

Season cracking results from the conjoint action of three components: (1) a susceptible material (copper and its alloys). (2) a specific chemical species (ammonia). (3) tensile stress (residual). For example Copper and its alloys are susceptible to ammonia compounds, Mild steels are susceptible to alkalis Stainless steels are susceptible to chlorides.

Mechanism Various models have been proposed which include the following.

Adsorption model: Specific chemical species adsorbs on the crack surface and lowers the fracture stress. Film rupture model: Stress ruptures the passive film locally and sets up an active-passive cell. Newly formed passive film is ruptured again under stress and the cycle continues until failure. Pre-existing active path model: Pre-existing path such as grain boundaries where intermetallics and compounds are formed. Embrittlement model: Hydrogen embrittlement is a major mechanism of environemntal cracking for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.

Environmental cracking can be prevented Control of stress level (residual or load) and hardness. Remove critical environmental species such as hydroxides, chlorides, and oxygen; Use of materials known not to crack in the specified environment. Control temperature and or potential choosing the right materials Avoid stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions

Factors Associated Mainly with the Metal Effective electrode potential of a metal in a solution. Overvoltage of hydrogen on the metal. Chemical and physical homogeneity of the metal surface. Inherent ability to form an insoluble protective film. Factors Which Vary Mainly with the Environment Hydrogen-ion concentration (pH) in the solution. Influence of oxygen in solution adjacent to the metal. Specific nature and concentration of other ions in solution. Rate of flow of the solution in contact with the metal. Ability of environment to form a protective deposit on the metal. Temperature. Cyclic stress (corrosion fatigue). Contact between dissimilar metals or other materials as affecting localized corrosion.

Method for removing troublesome ions from solution. a) Heavy metal trap: solutions containing copper ions enter barrel filled with aluminum shavings; b) Waster section: aluminum clad pipe inserted in a system removes heavy metal ions. The section is replaced once corroded.