Corrosion is the destruction of the surface of materials as a result of actively undergoing redox processes. The destruction of the layers of the material leads to a decrease in strength, electrical conductivity, increase fragility and inhibition of other properties of the metal.
During the operation of metal products, they are subjected to destructive influences of various types and types, among which pitting corrosion stands out. She is the most dangerous and unpredictable.
Pitting corrosion
On the surface of metal products quite often you can notice small indentations, points of brown or brown color. Scientists call such points pitting, and the process of their appearance is called pitting corrosion. It occurs on the surface of materials in contact with sea water, solutions of various salts, chemically aggressive environments and perceiving other negative factors.
Pitting corrosion affects only passive metals and alloys; it develops mainly in the anticorrosion layer or in places of various defects. "Point ulcers" can disrupt the operation of various products: from thin membranes and microcircuits, to thick-walled units. In addition, their appearance contributes to the formation of corrosion cracks, significantly reducing the specified characteristics of the material.
Metal fracture pattern
To activate pitting corrosion, the presence of two reagents is necessary - activators and passivators. The anions of chlorine, bromine, iodine most often act as activators - they are found in most environments in which metal products are operated. They are adsorbed on the surface of the metal and form soluble complexes with its components.
Most often, water or a hydroxyl group acts as passivators. Directly, the destruction process proceeds according to the following scheme:
- Activator ions are adsorbed on the surface of the protective (oxide) film.
- The process of replacing oxygen ions by ions of the activator of the process.
- A large amount of soluble ions is formed, as a result of which the film is destroyed.
As a result of this, a potential difference arises on the surface of the material, which leads to the appearance of local currents, and a violent anode process is activated. Activator ions in this case move to the foci of destruction, due to which pitting corrosion progresses.
Varieties of pitting corrosion
The type of pitting corrosion varies depending on environmental conditions, mainly on temperature, acidity, chemical composition of substances. Under the influence of these factors, the shape, size of pits and their location changes. So, according to the size, point destruction is distinguished:
- microscopic - point size less than 0.1 mm;
- usual - the diameter of the pits varies from 0.1 to 1 mm;
- ulcerative, when formations exceed 1 mm in diameter.
Depending on the location, pitting corrosion can be open or closed. In the first case, it is almost impossible to detect traces of destruction - it is necessary to use special devices. This type of corrosion very often leads to breakdowns.
Open rusting is noticeable to the naked eye. Often pitts merge into a single entity. In this case, the destruction of the material does not occur in depth, but in breadth, due to which large area defects arise.
Pitting Form
The shape of the pits depends on the voids inside the crystal lattice, which are formed in the first stages of the corrosion process. Most often, irregular formations are found - they occur on the surface of stainless, low alloy and carbon steels, aluminum, chromium, nickel alloys, and iron.
Hemispherical ulcers form as a result of isotropic dissolution. This process is similar to electropolishing. This partly explains the shiny bottom of the semicircular recesses. Titanium, aluminum, nickel and cobalt products, as well as tantalum structures, are most susceptible to such destruction. Pitting corrosion of stainless steels is approximately the same .
In addition, pits can be polyhedral and faceted. “Ulcers” of the latter type very often combine with each other, which leads to the appearance of large hemispherical destruction.
Reasons for the appearance
The main reasons for the appearance of pitting corrosion are a violation of production technology and mechanical impact on the material. As a result of a violation of casting technology, various microinclusions appear in the metal that violate its structure. The most common inclusion is mill scale.
Due to mechanical stress, pitting corrosion begins to develop very often on the surface of products. The reasons for this lie in the destruction of the upper protective film, the violation of the internal structure, and the emergence of grain boundaries on the surface. The most common activating factor can be called a dynamic effect, which leads to the appearance of microcracks.
Pitting corrosion of metals develops faster on rough surfaces, as well as under the influence of aggressive environments - sea water, acid solutions.
Methods of protecting metal from pitting corrosion
To protect metal products from pitting corrosion, three main methods are used:
- Elimination of closed systems using solutions of alkaline compounds, sulfates, chromates.
- Introduction to the composition of the material of components with high resistance to spot rusting - molybdenum, chromium, silicon.
- The use of cathodic and anodic technology for creating a protective layer.
All the presented methods of protecting metals from corrosion are applicable only in production, because they require high-tech equipment and large investments. In everyday life, it is impossible to completely eliminate the risk of pitting. It is only possible to weaken the influence of negatively acting factors by:
- applying anti-corrosion coatings ;
- improving product operating conditions;
- reduce the level of acidity of the medium with which the material is in contact.
But the most effective and affordable method is thorough polishing: by reducing the surface roughness, you simultaneously increase its corrosion resistance. But for the best effect, it is better to use all methods of protecting metals from corrosion at the same time.