Editing Corrosion (section)

== Corrosion in passivated materials ==
Passivation is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered.  Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials.  If breakdown occurs in the passive film due to chemical or mechanical factors, the resulting major modes of corrosion may include [[pitting corrosion]], [[crevice corrosion]], and [[stress corrosion cracking]].

=== Pitting corrosion ===
{{Main|Pitting corrosion}}
[[File:Pitting corrosion-scheme.png|thumb|upright|Diagram showing cross-section of pitting corrosion]]
Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which compete as [[anion]]s, can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause ''corrosion pits'' of several types, depending upon conditions. While the corrosion pits only [[nucleation|nucleate]] under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an autocatalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause [[stress concentration]] to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure [[structural failure|fails]]. Pitting remains among the most common and damaging forms of corrosion in passivated alloys,<ref>{{cite web|url= https://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm|title= Different Types of Corrosion: Pitting Corrosion - Causes and Prevention|author= <!--Not stated-->|date= |website= corrosionclinic.com|publisher= WebCorr Corrosion Consulting Services|access-date= 2022-09-13|quote= |archive-date= 2022-09-13|archive-url= https://web.archive.org/web/20220913050419/https://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm|url-status= live}}</ref> but it can be prevented by control of the alloy's environment.

Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure. This form of corrosion is often difficult to detect because it is usually relatively small and may be covered and hidden by corrosion-produced compounds.

=== Weld decay and knifeline attack ===
{{Main|Intergranular corrosion}}
[[File:Unsensitised structure of type 304 stainless steel.jpg|thumb|Normal microstructure of Type 304 stainless steel surface]]
[[File:Sensitized structure of 304 stainless steel.jpg|thumb|Sensitized metallic microstructure, showing wider intergranular boundaries]]
Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a major alloying component ([[chromium]], at least 11.5%). Because of the elevated temperatures of [[welding]] and heat treatment, [[chromium carbide]]s can form in the [[crystallite|grain boundaries]] of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a [[galvanic corrosion|galvanic couple]] with the well-protected alloy nearby, which leads to "weld decay" (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments. This process can seriously reduce the mechanical strength of welded joints over time.

A stainless steel is said to be "sensitized" if chromium carbides are formed in the microstructure. A typical microstructure of a normalized [[SAE 304 stainless steel|type 304 stainless steel]] shows no signs of sensitization, while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.

Special alloys, either with low carbon content or with added carbon "[[getter]]s" such as titanium and [[niobium]] (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of "knifeline attack". As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometers across, making it even less noticeable.

=== Crevice corrosion ===
{{Main|Crevice corrosion}}
[[File:Crevice corrosion of 316 stainless steel in desalination.jpg|thumb|Corrosion in the crevice between the tube and tube sheet (both made of [[Marine grade stainless|type 316 stainless steel]]) of a heat exchanger in a seawater desalination plant]]
[[Crevice corrosion]] is a localized form of corrosion occurring in confined spaces (crevices), to which the access of the working fluid from the environment is limited. Formation of a differential aeration cell{{explain|date=February 2023}} leads to corrosion inside the 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.

Crevice corrosion is influenced by the crevice type (metal-metal, metal-non-metal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature is commonly used to rank a material's resistance to crevice corrosion.

=== Hydrogen grooving ===
In the [[chemical industry]], hydrogen grooving is the corrosion of piping at grooves created by the interaction of a corrosive agent, corroded pipe constituents, and [[hydrogen]] gas [[Bubble (physics)|bubble]]s.<ref>{{cite web |title=The effect of sulphuric acid on storage tanks |url=https://www.sulphuric-acid.com/TechManual/Storage/storagetanks.htm |access-date=2019-10-27 |archive-date=2019-09-13 |archive-url=https://web.archive.org/web/20190913230920/http://www.sulphuric-acid.com/TechManual/Storage/storagetanks.htm |url-status=live }}</ref> For example, when [[sulfuric acid]] ({{chem2|H2SO4}}) flows through steel pipes, the [[iron]] in the steel reacts with the acid to form a [[Passivation (chemistry)|passivation]] coating of [[iron sulfate]] ({{chem2|FeSO4}}) and hydrogen gas ({{chem2|H2}}). The iron sulfate coating will protect the steel from further reaction; however, if hydrogen bubbles contact this coating, it will be removed. Thus, a groove can be formed by a travelling bubble, exposing more steel to the acid, causing a [[vicious cycle]]. The grooving is exacerbated by the tendency of subsequent bubbles to follow the same path.