Rouging, a form of (uniform corrosion)
Corrosion can basically be referred to as an undesired, thermodynamically induced, chemical alteration of the surface of a component (in this case: one made of a stainless steel alloy).
In stainless steel alloying technology, a wide range of different local and uniform corrosive effects, resulting from various different causes and mechanisms having more or less serious consequences for the component, is known.
In the case of rouging effects on a stainless steel surface, the phenomenon is a massive alteration in, serious damage to, or even inversion of the chromium-oxide-rich passive layer due to the influence of oxygen-deficient ultrapure water at temperatures > 60 °C, or ultrapure steam. These changes then result in a predominantly iron oxide rouging layer, which can be partially wiped away. The uniform corrosion process described above is referred to as 'rouging' in the related technical literature, which is a term derived from the reddish (Feoxide-rich) material particles that can usually be wiped away.
Technically speaking, rouging describes the typical golden-yellow to reddish-brown formation of film/discolouration on stainless steel surfaces with long-term exposure to hot, oxygen-deficient, salt-free water.
In clean-steam systems with high temperatures (T > 100 °C), it is even possible to observe rouging effects in the form of dark brown to violet films that normally stubbornly adhere to the surface. Relevant material analyses have shown that the rouge film formed on the stainless steel surface is essentially composed of heavy metal oxide particles (e.g. iron, chromium, nickel, etc.), with the iron oxide content clearly being predominant. As a result, it is safe to assume that—both with regard to their type and their relation to the composition—rouging deposits are a corrosion product of the specific stainless steel alloy.
图7：红锈层的形成、（Formation of a rouge layer , ）
From a thermodynamic standpoint, the che- mical process behind the entire rouging process is thus an advanced state of oxidation of the stainless steel material, or of its metal atoms, directly on the surface that has come into contact with media, a state that goes well beyond a passivation reaction.
In combination with pure, oxygen-deficient water or clean steam as environmental conditions for austenitic stainless steel surfaces in qualities of 1.4301/1.4404/1.4435/ 1.4571 etc., high process temperatures (> 60 °C) have clear negative effects on the morphological structure or natural repassivation property of stainless steel surfaces (= self-regeneration of the passive-layer conditions).
In addition to the oxygen depletion in the water, high water temperatures of up to 100 °C also cause an additional decompo- sition (dissociation) of water molecules accompanied by the formation of H+ and OH-ions. As a result of the increased formation of iron hydroxide, this in turn has a negative influence on the stability of the passive layer. At the same time, the oxygen deficiency causes not only a depassivation, but also prevents any further adequate repassivation (= oxidation of chromium), whereby a critical shift in the dynamic balance of deand repassivation occurs, with this balance clearly tipping towards depassivation.
Decomposition of water molecules as temperature increases Blue curve: Decrease in oxygen content Green curve: Increase in water decomposition (formation of ions) .
Taken together, this leads to a noticeable reduction in the passive layer's effectiveness and, over time, ultimately to the formation of the predominately iron oxide rouging layer, due to the systematic breakdown/decomposition of the closed protective chromium oxide layer. In the process, the impact of the iron-dominated base material is magnified. In terms of potential, these changes essentially amount to an initially local and eventually complete depassivation.
Due to its bonding energies, iron exhibits— particularly at higher temperatures—a higher affinity for the free hydroxide ions (of the water) that are present as a result of the temperature-induced increased decomposition of the water. More iron hydroxide forms on the activated stainless steel surface as a precursor of the iron(II) and iron(III) oxide.
In a thermodynamically induced phase inversion, the predominantly chromium oxide protective layer, which was initially present as a passive surface, is converted into a layer rich in iron oxide. This new iron oxide layer (= rouging layer) forms a cover which has a microporous, layered structure that is typical of rusty steel surfaces. The mechanism behind the production/formation of rouging is essentially a form of uniform corrosion. In many cases, no distinction can be made between the corrosion products of rouging and the uniform corrosion due to completely different mechanisms.
Compared to other types of corrosion, typical rouging is relatively easy to identify, usually by means of a simple swab test (see Figure 9).
图9：红锈覆盖的不锈钢表面的拭子样品（Swab sample from a stainless steel surface covered by rouging ）
In many application cases—especially in steam sterilizers—the occurrence of typical rouging films on the component surface, and the accompanying significant discolouration, cannot be avoided due to the combination of high water or water vapour temperatures, oxygen deficiency and the (thermodynamic) property of the specific material (= defined stainless steel alloy).
In light of these circumstances, operators need to specify preventative and restorative measures in order to create corrosion-resistant passive surface conditions.