 | This article needs additional citations for verification .(January 2008) |
Crevice corrosion refers to corrosion occurring in occluded spaces such as interstices in which a stagnant solution is trapped and not renewed. [1] These spaces are generally called crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. [2] [3]
The corrosion resistance of a stainless steel is dependent on the presence of an ultra-thin protective oxide film (passive film) on its surface, but it is possible under certain conditions for this oxide film to break down, for example in halide solutions or reducing acids. [4] Areas where the oxide film can break down can also sometimes be the result of the way components are designed, for example under gaskets, in sharp re-entrant corners or associated with incomplete weld penetration or overlapping surfaces. [5] These can all form crevices which can promote corrosion. To function as a corrosion site, a crevice has to be of sufficient width to permit entry of the corrodent, but narrow enough to ensure that the corrodent remains stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not found in grooves or slots in which circulation of the corrodent is possible. This problem can often be overcome by paying attention to the design of the component, in particular to avoiding formation of crevices or at least keeping them as open as possible. Crevice corrosion is a very similar mechanism to pitting corrosion; alloys resistant to one are generally resistant to both. Crevice corrosion can be viewed as a less severe form of localized corrosion when compared with pitting. The depth of penetration and the rate of propagation in pitting corrosion are significantly greater than in crevice corrosion.
Crevices can develop a local chemistry which is very different from that of the bulk fluid. For example, in boilers, concentration of non-volatile impurities may occur in crevices near heat-transfer surfaces because of the continuous water vaporization. "Concentration factors" of many millions are not uncommon for common water impurities like sodium, sulfate or chloride ions. [6] The concentration process is often referred to as "hideout" (HO), whereas the opposite process, whereby the concentrations tend to even out (e.g., during shutdown) is called "hideout return" (HOR). In a neutral pH solution, the pH inside the crevice can drop to 2, a highly acidic condition that accelerates the corrosion of most metals and alloys.
For a given crevice type, two factors are important in the initiation of crevice corrosion: the chemical composition of the electrolyte in the crevice and the electrical potential drop into the crevice. Researchers had previously claimed that either one or the other of the two factors was responsible for initiating crevice corrosion, but recently it has been shown that it is a combination of the two that causes active crevice corrosion. [7] Both the drop of potential and the change in composition of the crevice electrolyte are produced by the oxygen depletion of the solution inside the crevice (oxygen consumption caused by the metal oxidation at the inner surface of the occluded cavity) and the separation of electroactive areas, with net anodic reactions (oxidation) occurring within the crevice and net cathodic reactions (reduction) occurring at the exterior of the crevice (on the bold surface). The ratio of the surface areas between the cathodic and anodic region is significant.
Some of the phenomena occurring within the crevice may be somewhat reminiscent of galvanic corrosion:
The mechanism of crevice corrosion can be (but is not always) similar to that of pitting corrosion. However, there are sufficient differences to warrant a separate treatment. For example, in crevice corrosion, one has to consider the geometry of the crevice and the nature of the concentration process leading to the development of the differential local chemistry. The extreme and often unexpected local chemistry conditions inside the crevice need to be considered. Galvanic effects can play a role in crevice degradation.
Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form of:
A common form of crevice failure occurs due to stress corrosion cracking, where a crack or cracks develop from the base of the crevice where the stress concentration is greatest. This was the root cause of the fall of the Silver Bridge over the Ohio River, in 1967 in West Virginia, where a single critical crack only about 3 mm long suddenly grew and fractured a tie bar joint. The rest of the bridge fell in less than a minute. The disaster was caused by one single point of failure (SPOF).
The eyebars in the Silver Bridge were not redundant, as links were composed of only two bars each, of high strength steel (more than twice as strong as common mild steel), rather than a thick stack of thinner bars of modest material strength "combed" together as is usual for redundancy. With only two bars, the failure of one could impose excessive loading on the second, causing total failure—unlikely if more bars are used. While a low-redundancy chain can be engineered to the design requirements, the safety is completely dependent upon correct, high quality manufacturing and assembly.
The susceptibility to crevice corrosion varies widely from one material-environment system to another. In general, crevice corrosion is of greatest concern for materials which are normally passive metals, like stainless steel or aluminum. Crevice corrosion tends to be of greatest significance to components built of highly corrosion-resistant superalloys and operating with the purest-available water chemistry. For example, steam generators in nuclear power plants degrade largely by crevice corrosion.
Crevice corrosion is extremely dangerous because it is localized and can lead to component failure while the overall material loss is minimal. The initiation and progress of crevice corrosion can be difficult to detect.
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.
Stainless steel, also known as inox, corrosion-resistant steel (CRES) and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains at least 10.5% chromium and usually nickel, as well as 0.2 to 2.11% carbon. Stainless steel's resistance to corrosion results from the chromium, which forms a passive film that can protect the material and self-heal in the presence of oxygen.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.
Plating is a finishing process in which a metal is deposited on a surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.
Sulfide stress cracking (SSC) is a form of hydrogen embrittlement which is a cathodic cracking mechanism. It should not be confused with the term stress corrosion cracking which is an anodic cracking mechanism. Susceptible alloys, especially steels, react with hydrogen sulfide (H2S), forming metal sulfides (MeS) and atomic hydrogen (H•) as corrosion byproducts. Atomic hydrogen either combines to form H2 at the metal surface or diffuses into the metal matrix. Since sulfur is a hydrogen recombination poison, the amount of atomic hydrogen which recombines to form H2 on the surface is greatly reduced, thereby increasing the amount of diffusion of atomic hydrogen into the metal matrix. This aspect is what makes wet H2S environments so severe.
Marine grade stainless alloys typically contain molybdenum to resist the corrosive effects of NaCl or salt in seawater. Concentrations of salt in seawater can vary, and splash zones can cause concentrations to increase dramatically from the spray and evaporation.
Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.
In materials science, intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.
In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst equation. A concentration cell produces a small voltage as it attempts to reach chemical equilibrium, which occurs when the concentration of reactant in both half-cells are equal. Because an order of magnitude concentration difference produces less than 60 millivolts at room temperature, concentration cells are not typically used for energy storage.
Michael A. Streicher was an American metallurgist and engineer who became internationally recognized for his work on the testing and development of corrosion-resistant stainless steel alloys. He published widely in technical journals and textbooks and received numerous patents for his inventions.
Corrosion engineering is an engineering specialty that applies scientific, technical, engineering skills, and knowledge of natural laws and physical resources to design and implement materials, structures, devices, systems, and procedures to manage corrosion. From a holistic perspective, corrosion is the phenomenon of metals returning to the state they are found in nature. The driving force that causes metals to corrode is a consequence of their temporary existence in metallic form. To produce metals starting from naturally occurring minerals and ores, it is necessary to provide a certain amount of energy, e.g. Iron ore in a blast furnace. It is therefore thermodynamically inevitable that these metals when exposed to various environments would revert to their state found in nature. Corrosion and corrosion engineering thus involves a study of chemical kinetics, thermodynamics, electrochemistry and materials science.
Materials for use in vacuum are materials that show very low rates of outgassing in vacuum and, where applicable, are tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the vacuum chamber. The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed on the material surface. Materials may sublimate in vacuum. Or the gases can be released from porous materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.
A sacrificial metal is a metal used as a sacrificial anode in cathodic protection that corrodes to prevent a primary metal from corrosion or rusting. It may also be used for galvanization.
Pitting resistance equivalent number (PREN) is a predictive measurement of a stainless steel's resistance to localized pitting corrosion based on its chemical composition. In general: the higher PREN-value, the more resistant is the stainless steel to localized pitting corrosion by chloride.
SAE 304 stainless steel is the most common stainless steel. It is an alloy of iron, carbon, chromium and nickel. It is an austenitic stainless steel, and is therefore not magnetic. It is less electrically and thermally conductive than carbon steel. It has a higher corrosion resistance than regular steel and is widely used because of the ease in which it is formed into various shapes.
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices. This phenomenon is named after Italian physician Luigi Galvani (1737–1798).