Types of corrosion
Intergranular corrosion is defined as an attack along the grain boundaries, while the grains themselves are not or hardly removed. The attack of the grain boundaries can go so far that individual grains are detached from the grain structure, causing the steel to lose its cohesion. The cause of intergranular corrosion in stainless steels is precipitation of chromium-rich carbides at the grain boundaries, which results in chromium depletion in the areas near the grain boundaries.
The chromium-poor zones formed in this way at the grain boundaries are not sufficiently corrosion-resistant against most aggressive media and therefore dissolve very quickly. Chromium carbide precipitation presupposes a certain carbon content and takes place in the temperature range between 500 and 80°C, which is passed through, for example, during heat treatment or welding processes. To avoid chromium carbide precipitation, the carbon content in stainless steels can be reduced to below 0.03% or the carbon present can be bound by so-called stabilizing elements such as titanium (Ti) or niobium (Nb), which have a greater carbon affinity than chromium.
If chromium precipitations have occurred, these can be dissolved again at solution annealing temperatures above 1050°C. In the case of unstabilized ferritic steels, existing susceptibility to intergranular corrosion can be eliminated by annealing at 800 - 85 °C. In this case, chromium depletion in the grains is eliminated by post-diffusion from the interior of the grains. In this process, chromium depletion in the grain boundary areas is eliminated by post-diffusion of chromium from the grain interior.
Pitting and crevice corrosion
In practice, pitting and crevice corrosion are mostly caused by chloride ions (Cl-). In addition, the less frequently encountered halides bromide (Br-) and iodide (l-) can also be triggers. Pitting corrosion is initiated by an interaction between the halide ions and the passive layer, whereby the passive layer is locally breached. Pinprick-like depressions are formed and, as a result of their growth, pitting corrosion sites, which can have very different characteristics. The risk of pitting corrosion increases with
- increasing concentration of halide ions
- increasing temperature
- increase in the electrochemical potential of the steel in the electrolytes concerned, caused, for example, by the action of an oxidizing agent
Crevice corrosion occurs in crevices where fluid exchange with the environment is restricted. Such crevices are construction- or operation-related and are found, for example, in flanges, on pipe roll-ins, under seals or also under incrustations. The corrosion mechanism is essentially the same as that of pitting corrosion. Additional influencing factors are the gap geometry and the type of gap-forming materials. Since crevice corrosion occurs at significantly lower corrosion stresses than pitting corrosion, the occurrence of crevices in chloride-containing media should be avoided as far as possible by design measures. If the alloying element distribution is homogeneous, the relative pitting and crevice corrosion resistance of a stainless steel can be estimated approximately via the effective sum (W): W = %Cr + 3.3 x %Mo + 30 x %N. However, the influence of the alloying element nitrogen (N) is more complex than this relationship expresses. The high efficiency expressed in the factor of 30 is likely to be fully effective only for high-alloy steels with elevated molybdenum contents. Nonmetallic impurities, especially sulfide precipitates, promote pitting and crevice corrosion when they come to the surface. A surface that is as smooth as possible can be advantageous. It makes it more difficult for deposits to adhere, leading to crevice corrosion conditions. A high resistance to pitting and crevice corrosion is only achieved with a perfect surface finish, i.e. a bright metallic surface. Therefore, tarnish and scale residues after welding, foreign iron abrasion, extraneous rust, abrasive residues, etc. must be removed (see pickling).
This form of corrosion occurs on metal parts in the presence of a corrosive medium in unsealed support gaps such as overlaps, attached webs and in welds that are not fully welded. A distinction is made in crevice corrosion between oxygen and hydrogen types. In both cases, the driving force is the concentration difference between the gap and the outer gap area of the corrosive medium. The potential difference associated with the concentration difference leads to electrochemical corrosion of the gap (hydrogen type) or its immediate surroundings (oxygen type).
Even stainless steel can corrode in crevices if oxygen cannot be present there to form the protective oxide layer. A corrosion-resistant selection of the material of construction is initially the most economical way of preventing corrosion. Furthermore, crevice corrosion can be prevented by avoiding crevices in the design or by making them so large that crevice corrosion is avoided. In addition, gaps that are unavoidable in the design can be sealed with plastics; this principle is used in the connection of bolts and nuts that are at risk of corrosion, which are secured against unintentional loosening and protected against crevice corrosion by an appropriate adhesive.
External rust is defined as deposits of rust particles which did not originate at the site in question but were carried in from somewhere else. External rust occurs preferentially when "black" and "white" steel are not stored and processed separately. But tool abrasion can also lead to extraneous rust. Deposits of external rust can fulfill the conditions for crevice corrosion.
Stress corrosion cracking
Media with specifically acting components - especially chloride ions - can lead to a corrosion attack with crack formation in stainless steels if tensile stresses are applied simultaneously, even if the steel is sufficiently resistant without mechanical stress in the medium. This phenomenon, known as stress corrosion cracking, can be triggered not only by externally applied tensile stresses. Frequently, the cause is also inherent stresses introduced during processing, such as welding, grinding or cold forming.
As with pitting and crevice corrosion, the risk of chlorine-induced stress corrosion cracking increases with rising temperature and chloride concentration. On the materials side, however, other influencing variables are effective. For example, austenitic steels of the 18/10 - CrNi and 18/10/2 - CrNiMo types are particularly at risk from chlorine-induced stress corrosion cracking at temperatures above approx. 50°C. However, by increasing the molybdenum and especially the nickel content, the resistance can be increased quite considerably. Ferritic and ferritic-austenitic stainless steels are also comparatively less sensitive.
Vibration Corrosion Cracking
Vibration corrosion cracking is the transcrystalline or intercrystalline cracking in materials under the causal influence of vibrations. The occurrence of this corrosion depends on the stress and usually occurs above a specific limit value. Vibration crack corrosion occurs suddenly and is usually not visible externally. The fatigue strength of all NIROSTA® steels is reduced to a greater or lesser extent by additional chemical attack. The decrease in fatigue strength depends, apart from the attack agent, on the multiaxiality of the alternating stresses that occur.
The possibility of contact corrosion exists when two metals with different free corrosion potentials are conductively connected to each other in a corrosion medium. The metal with the lower free corrosion potential is conductively connected to each other. The metal with the lower free corrosion potential can at least be polarized towards higher potentials and thus be attacked more strongly. Even with large differences between the free corrosion potentials of the metals involved, however, contact corrosion does not necessarily occur. This depends on the electrochemical behavior of the two metals. The conductivity of the medium and the surface behavior of the metals involved are also important. If the less noble metal has a much larger surface area than the more noble one, and the corrosion medium has a high conductivity, the risk of corrosion damage is lower. However, the combination of a base metal with a small surface area and a noble metal with a large surface area should be avoided. Stainless steels generally take up high free corrosion potentials and are therefore hardly subject to the risk of increased attack by contact corrosion. However, it is much more common for other metals with lower free corrosion potential to be subject to contact corrosion as a result of bonding with a stainless steel.
High temperature corrosion
High-temperature corrosion is a chemical process which, in the absence of free water (as in the case of wet corrosion), can lead to a reduction in the durability of materials. The triggering process is not the water, but the high temperature to which a material is exposed, taking into account other environmental conditions such as the type or composition of the medium, media velocities and density, number and size of particles and the like. The damage pattern is similar to that of wet corrosion. Basically, all possible forms of corrosion, such as uniform surface corrosion, pitting corrosion, contact corrosion, etc., can occur. The occurrence of hydrogen embrittlement can also be a common hazard. The speed of the process taking place can be slowed down by the formation of cover layers. For example, scaling (oxidation by oxygen) can be greatly impaired by alloying the material with aluminum, silicon and especially chromium. These alloying elements in fact form very dense oxide layers which effectively impede the diffusion-controlled process of scaling.
Other atmospheres of importance in high-temperature corrosion besides oxygen are:
- Sulfur (S)
(Atmospheres in which SO2 and water are present (e.g. flue gases from a coal-fired power plant) may have very damaging effects on the base material. The components just mentioned can react to form sulfuric acid. If the temperature falls below the dew point (sulfuric acid condenses at approx. 200°C), a sulfuric acid condensate then forms on the material, which attacks it very severely).
- Nitrogen (N)
(N2 can diffuse into the material and form nitrides with certain components. (N2 can diffuse into the material and form nitrides with certain components, which can have a lasting negative effect on the mechanical properties of the component).
- Sodium (Na)
(Na can sometimes form a low-melting eutectic system together with the base material. This process leads to rapid destruction of the component).
Uniform surface corrosion occurs when the corrosion process occurs at many points on a component. Anodic (metal dissolving) and cathodic (electron consuming) partial areas form on the metal surface. A prerequisite for the formation of surface corrosion is the constant change of location of these partial areas. Only then can the corrosion proceed uniformly on the metal surface. If this change of location of the sub-areas is not possible or only possible with difficulty, we speak of pitting corrosion.
In the case of surface corrosion in neutral areas, a covering layer (passive layer) is formed on the metal, which greatly reduces the susceptibility to corrosion, or prevents or restricts the corrosion itself. If the passive layer has not formed in some areas, or if it has been destroyed by acid attack (enhanced by anions of chlorine), this also leads to pitting corrosion / pitting corrosion.
Surface corrosion can be classified as rather harmless corrosion, since it can be detected at an early stage, and can also lead to damage only when the mass loss of the metal is sufficiently large. If the intensity of subsequent exposure to atmospheric gases, acids, etc. is known, the sacrificial layer can be adequately dimensioned . In this way, the safety of a component is also guaranteed in the longer term.
Stainless steels are characterized by special resistance to chemically aggressive aqueous media. As a rule, they have a chromium (Cr) mass fraction of at least 12% and a carbon (C) mass fraction of at most 1.2%.
The high corrosion resistance of stainless steels is based on their ability to form a passive layer on the surface. This is a chromium-rich metal oxide or metal oxide hydrate layer only a few nm thick, which separates the metal from the attacking medium. The passive layer of a stainless steel is not something unchanging, but adjusts its composition and structure in equilibrium with the surrounding medium over time. Once a passive layer has formed, it cannot therefore be transferred to another medium. Passive layer formation, e.g. after mechanical damage to the surface (abrasion), generally occurs again by itself. If a sufficient passive layer cannot form in a medium, or if the existing passive surface layer is locally breached or completely destroyed by chemical means, corrosion damage can occur.
The alloying element that is decisive for the ability to form a passive layer is chromium (Cr). Chromium contents above the above-mentioned value of approx. 12% suppress rust formation under normal atmospheric corrosion stress. By further increasing the chromium content and - depending on the application - adding molybdenum (Mo) and also other alloying elements, the resistance can be extended to much more aggressive conditions. Only the content of alloying elements dissolved in the metal is effective for passivation. Therefore, the highest corrosion resistance in each case is exhibited by a segregation-free matrix that is not depleted by precipitation or formation of intermetallic phases of, for example, chromium and molybdenum.
The correct heat treatment to achieve an optimum microstructure is described in the relevant material data sheets. Stainless steels are susceptible to ablative surface corrosion and various forms of localized corrosion, which are discussed in more detail below.
The steel surface exposed to chemical attack must be as smooth as possible and free of impurities of any kind. Foreign matter pressed into the surface during machining, such as abrasive residues or tool debris, considerably reduces corrosion resistance. Nonmetallic contaminants, especially sulfide precipitates, promote localized corrosion when they come to the surface.
Mechanical surface treatment
Mechanical surface treatments may be required for various reasons. One is to remove tarnish after welding or after heat treatment. On the other hand, mechanical post-treatment can also be carried out for purely visual reasons to achieve a certain surface effect. When grinding austenitic stainless steels, it should be noted that their thermal conductivity is lower than that of unalloyed or ferritic stainless steels. In order to avoid local overheating and thus slight tarnishing and warping during grinding, the contact pressure must not be too high. Abrasives used for parts made of unalloyed steels should not be used for stainless steels as iron abrasion presses into the surface and leads to the formation of extraneous rust. In addition, care must be taken to ensure that the abrasives are free of iron and sulfur in order to prevent corrosion and extraneous rust.
In addition, under certain conditions, sand or glass bead blasting is used as a pretreatment for downstream electroplating (a substitute for pickling) or to remove scale on hot-formed or heat-treated fasteners.
Chemical surface treatment
Pickling of stainless steels is often a compelling necessity to remove the scale that forms during heat treatment or the tarnish that forms during welding. Chemical surface treatment is either carried out in pickling baths or by means of pickling pastes. Pickling pastes are mainly used to remove tarnish after welding, i.e. partially. Entire structures, vessels, etc., which have been subjected to heat treatment, are pickled almost exclusively to remove scale layers. Passivation accelerates the formation of the passive layer, which is generally formed already upon exposure to water or atmospheric oxygen and causes corrosion resistance of stainless steels. Passivation is therefore recommended, but often additionally not necessary, since the pickling baths and pickling pastes already contain the oxidizing acids. However, it is advisable to check with the manufacturers of pickling baths and pickling pastes. When pickling and passivating, it is essential to observe the safety regulations for working with acids as well as the regulations for water and environmental protection.
One of the common pickling solutions has the following composition:
- Nitric acid (50% by volume): 10 - 30 vol.
- Hydrofluoric acid: 2.5 - 3.0 % by volume
- Water: remainder
- Bath temperature: 20 - 40 °C
- Pickling time: depending on thickness and composition of scale
Electropolishing, also called chemical polishing (burnishing), is particularly suitable for parts that cannot be polished mechanically (e.g. complicated parts, thin-walled constructions or parts that bend easily). In electropolishing, the parts are suspended in a special bath. The parts to be polished are connected as an anode, which causes metallic removal of the surface.
Hydrogen-induced cracking (hydrogen embrittlement)
During pickling or cleaning processes and the electroplating of ferritic steel parts, atomic hydrogen is always deposited from the process bath and can diffuse into the steel surface.
In the steel, the atomic hydrogen migrates to zones of high tensile stress (outer and inner notches), accumulates there, and weakens the metal bond Reduction of cohesive forces between Fe molecules) until a micro-crack is formed.
As a result, this zone relaxes, but new stress concentrations are formed at the crack tip, which in turn attract atomic hydrogen again, are weakened, crack, and so on. This continues until the residual cross-section can no longer support the external tensile load and spontaneously fractures.
In addition, hydrogen can accumulate in internal pores, combine to form molecules, and thus generate very high pressures that form internal incipient cracks, possibly to the point of destruction.
In the case of local stress concentrations, preferably on cold-formed workpieces, cracks can form even in the unloaded state, which can finally lead to brittle fracture.
Parts with tensile strengths Rm < 1,000 MPa are generally not critical. All steel parts with a tensile strength of Rm ≥ 1,000 MPa are considered high-strength and are therefore considered critical.
One can try to minimize hydrogen absorption by appropriate process control (bright surfaces, shot blasting instead of pickling, inhibitors) and drive out some of the hydrogen by annealing (heating) the parts after electroplating.
This heat treatment must be performed no later than 4 hours after plating and can take from six to 24 hours at approximately 210°C (± 10°C), depending on tensile strength.
It should be noted, however, that the temperature-supported hydrogen expulsion (de-brittling) cannot reverse a brittle fracture that has already set in. Subsequent heat treatment reduces the risk of hydrogen embrittlement, but complete elimination cannot be guaranteed. The residual risk is borne by the customer.
An embrittlement test can only be carried out by a stress test (as high tensile stresses as possible) for 24 - 96 hours at 20°C, during which no part may break.
For critical parts, an alternative coating (organic/inorganic) or the use of stainless steel is recommended.
DIN EN ISO 15330 - Fasteners - Stressing test for detection of hydrogen embrittlement - Method with parallel bearing surfaces.
DIN EN ISO 4042 - Fasteners - Electroplated coatings