Pitting corrosion: causes. Methods for protecting metals from corrosion

Corrosion is the destruction of the surface of materials as a result of actively passing redox processes. The destruction of layers of material leads to a decrease in strength, electrical conductivity, increased brittleness and oppression of other properties of the metal.

During the operation of metal products, they are subjected to destructive effects of various types and types, among which pitting corrosion is distinguished. It is the most dangerous and unpredictable.

Pitting Corrosion

On the surface of metal products, you can often notice small depressions, points of brown or brown color. Such points scientists call pitting, and the process of their appearance - pitting corrosion. It occurs on the surface of materials in contact with seawater, solutions of various salts, chemically aggressive media and other negative factors.

Pitting corrosion affects only passive metals and alloys, develops primarily in the anticorrosion layer or in the places of various defects. "Punctual ulcers" can disrupt the work of various products: from thin membranes and microcircuits, to thick-walled aggregates. In addition, their appearance contributes to the formation of corrosion cracks, significantly reducing the specified characteristics of the material.

Metal Breakdown Scheme

For the activation of pitting corrosion, the presence of two reagents - activators and passivators. As an activator, chlorine, bromine and iodine anions are most often present - they are found in most media in which metal products are used. They are adsorbed on the metal surface and form soluble complexes with its components.

As passivators, water or a hydroxyl group most often appears. Directly the process of destruction proceeds according to the following scheme:

1. Ion-activators are adsorbed on the surface of the protective (oxide) film.
2. There is a process of substitution of oxygen ions for the ions of the activator of the process.
3. A large amount of soluble ions is formed, as a result of which the film collapses.

As a result, a potential difference appears on the surface of the material, which leads to the appearance of local currents, the turbulent anode process is activated. Ion-activators move to the centers of destruction, due to which pitting corrosion progresses.

Varieties of pitting corrosion

The form of pitting corrosion varies depending on environmental conditions, mainly on the temperature, acidity, chemical composition of the substances. Under the influence of these factors, the shape, size of pittings and their arrangement change. So, according to the size, point destruction is distinguished:

• Microscopic - the size of points is less than 0.1 mm;
• Normal - the diameter of the pitting varies from 0.1 to 1 mm;
• Ulcerative, when the formations exceed 1 mm in diameter.

Depending on the location, pitting corrosion may be of an open or closed type. In the first case, it is practically impossible to detect traces of destruction - it is necessary to use special devices. This type of corrosion very often leads to the appearance of breakdowns.

Open rust is noticeable to the naked eye. Often pittings merge into a single entity. At the same time, the destruction of the material does not take place in depth, but in width, which causes large defects in the area.

Pitting Form

The shape of the pitting depends on the voids inside the crystal lattice, which are formed during the first stages of the corrosion process. The most common formations are irregular shapes - they arise on the surface of stainless, low-alloy and carbon steels, aluminum, chromium, nickel alloys, iron.

Hemispherical ulcers are formed as a result of isotropic dissolution. This process is similar to electropolishing. In part, this explains the brilliant bottom of the semicircular depressions. The titanium, aluminum, nickel and cobalt products, as well as tantalum constructions, are the most susceptible to such destruction. Approximately the same kind has pitting corrosion of stainless steels.

In addition, pitting can be polyhedral and faceted. "Ulcers" of the latter type very often unite with each other, which leads to the emergence of large hemispherical destruction.

Causes of appearance

The main reasons for the appearance of pitting corrosion are the violation of production technology and the mechanical effect on the material. As a result of the violation of casting technology, various microinclusions appear in the metal, which disrupt its structure. The most common inclusion can be called rolling scale.

Due to the mechanical action, pitting corrosion often begins to develop on the surface of the products. The reasons for this lie in the destruction of the upper protective film, the breakdown of the internal structure, the emergence of grain boundaries on the surface. The most common activating process is the 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 media - sea water, acid solutions.

Methods for protecting metal from pitting corrosion

Three main methods are used to protect metal articles from pitting corrosion:

1. Liquidation of closed systems with solutions of alkaline compounds, sulfates, chromates.
2. The introduction of components with high resistance to point rusting - molybdenum, chromium, silicon.
3. The use of cathodic and anodic technology to create a protective layer.

All the methods of protection of metals from corrosion are applicable only in production, because they require high-tech equipment and large capital 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:

• Application of anticorrosive coatings;
• Improvement of operating conditions of products;
• Decrease 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 roughness of the surface, you simultaneously increase its anti-corrosion resistance. But for best effect it is better to use all methods of protecting metals from corrosion at the same time.