Cover pitting and its dangers

Pitting pitting: what it is and how dangerous it is

People often claim that each metal can be protected with a special oxide coating that will inhibit corrosion.

But there is also a special kind of rust called pitting corrosion, which affects coated metals. In most cases, such corrosion will affect only the upper oxide metal layer, and will penetrate deeply rather slowly.

But how does this rust appear? Is it true that there is corrosion on stainless steel? Below you will find answers to all questions.

What is pitting

So, pitting is a type of corrosion in which so-called pitting will appear on the metal surface.

Such corrosion will affect copper, iron, aluminum alloys, as well as those based on chromium and others. Pitting type corrosion is possible even on stainless steel.

Pitting will usually involve different metal structures that will come into contact with salt water (usually different areas near the shore). This is due to the fact that for the reaction to take effect, an excess of activator ions is required, which will displace oxygen from the protective oxide film - and such substances are released in large quantities in water. Also note that pitting usually begins to affect the top layers of the oxide film at first, but due to the increasing spread of rust, it can begin to take over the metal completely.

Such corrosion usually occurs when two factors are combined at once.

Details

Reasons for occurrence

We suggest considering the main causes of pitting pitting corrosion.

• Mechanical deformation

This may be a scratch, dent, cracking in the impact area, and more. This factor will be key, because most metal alloys have a very thin layer of protection on the surface that helps prevent corrosion. Naturally, if such a layer is damaged, the metal will become unprotected against rust.

• Structural heterogeneity

This factor is also extremely important, because heterogeneities will often create small-sized foci, where after a while rust will begin to appear. Inexperienced engineers may think that such a factor will only be dangerous for low quality metal and steel, but this is not at all the case. Indeed, low quality alloys will have a heterogeneous structure and rust much more often, but structural heterogeneity can also appear in parts that are processed, on which they forgot to apply a protective coating. A simple example - while drilling a hole, the integrity of the outer anti-corrosion film will be compromised - this will draw attention to the appearance of damage.

• High degree of roughness

If the surface of the object is metal is very rough, then in such a case a special anti-corrosion coating will hardly be retained on it.

For this reason, the appearance of corrosion on such a surface, but this is only a matter of time. Also, pay attention to the fact that one rule will apply here - the greater the roughness of the metal surface, the sooner it will become covered with corrosion. A smooth, homogeneous metal will have excellent rust resistance.

• Aggressive environment

Do not believe it, but even pitting corrosion of stainless steel can occur if it comes into contact with aggressive media (acids, water with a huge content of salts, alkalis, etc. ).

Aggressive media can also damage the outer anti-corrosion layer, and this, over time, can lead to pitting. Also note that different substances will affect the metal in different ways - if seawater does not cause any damage during short-term contact, but if it comes into contact with strong acids, severe damage can appear. For this reason, the rules for storage and processing of metal should be observed.

Stages of pitting corrosion

The main danger of such corrosion will be its rapid spread. The whole point is that as rust appears, the protective outer layer will collapse, and therefore such corrosion cannot be localized in a certain area.

Even the smallest fragments grow and increase in size, and in the absence of timely treatment, corrosion will very quickly begin to completely seize the metal, which will make it useless and to some extent dangerous (for example, when it comes to a hinged metal structure).

Such corrosion on metal will spread in several stages:

1. Point rust begins to appear in places where the anti-corrosion coating is damaged (cracks, scratches, dents, etc.), as well as with an inhomogeneous metal structure. Another localization will be processed metal, which for some reason did not have time to apply a protective coating.
2. At the chemical level, this will happen like this - the activator ions under the influence of electrochemical force will displace oxygen from the oxide film, which is covered with a sheet of metal or a product.

   This will gradually erode the outer coating of the metal, producing characteristic sores and reddish-brown spots.

3. As the oxide film breaks down, rust begins to capture new areas of the surface of a metal object, this can lead to a deterioration in physical properties (loss of hardness / density / strength, etc.). The oxidation reaction will proceed according to the electrochemical scenario due to the displacement of oxygen from the oxide film.
4. After the complete destruction of the oxide film, the pitting will begin to penetrate into the metal alloy - and then the diffusion stage will begin.

   The rate of such a process will be quite low, and complete rusting can take a huge amount of time.

It is worth paying attention to the fact that sometimes spontaneous metal passivation will occur, which will slow down the appearance of rust. In practice, this scenario is very rare, although such cases can be found in practice. It is worth paying attention to the fact that in the case of transition of pitting corrosion to the diffuse stage, passivation will not be possible for physical reasons.

Classification

There are several types of pitting:

1. Surface.

   In such a scenario, rust will only start to affect the top metal surface. It will also spread in the form of small and thin lines with a diameter of 0.1 to 0.3 cm. Surface rust at the initial stage will usually capture the corners, but after a while it will begin to spread horizontally over the entire surface of the metal element.

2. Open. In such a scenario, corrosion will begin to appear in the form of frequent large points, and their diameter will be from 0.2 to 0.5 cm. On the surface, several points will first appear, which are located far from each other.

   Over time, the number of points will grow in direct proportion, and they will begin to capture the entire surface of the metal object.

3. Closed. In such a scenario, pitting will first capture the inner surface of the metal film, and corrosion propagation usually occurs in the form of short lines or large circles with an average diameter (from 0.5 to 1.5 cm).

   Such a scenario will be extremely rare, and is the most dangerous, as well as disastrous due to the complexity of timely detection. Such corrosion will manifest itself only at a late stage of corrosion growth, when the metal cannot be saved.

Now a little about how to protect objects.

Protection of metal objects

Passivation will be the main method of protecting stainless steel and metal from pitting corrosion. For processing, a special solution based on citric and nitric acid is usually used.

If necessary, the acid solution can be enhanced with various auxiliary additives. Certain engineers add ferrocyanide to the potassium solution at a concentration of 2.5%. The purpose of passivation will be to slow down corrosion until new rust has completely stopped. This acidic solution will act as a new protective film that will form on the surface during the process.

In addition, other auxiliary protective measures can be used:

• Sealing of defects and cracks. One of the main reasons for the appearance of corrosion will be the violation of the integrity of the oxide film due to external defects.If such damage is repaired in time, then rust will not have time to appear.
• Removal of roughness and unevenness. Rust appears very often on uneven surfaces, and scraping the metal surface will reliably protect the element.

• Chrome plating. Certain types of steel can be protected by applying an additional chromium-based coating, and such an element will also prevent the appearance of rust.

We hope that now it becomes easier for you to protect metal elements.

Conclusion

Pitting is a special form of rust that traps the oxide protective layer of the metal. In most cases the rust will spread in long streaks and small dots.

At a later stage, large, uneven spots and long ulcers-streaks may appear.

The main causes of pitting will be mechanical defects, chemical damage, the presence of irregularities, etc. Depending on the type of rust, several types of pitting will be distinguished - closed, open and surface. The main method of metal protection will be passivation, as well as timely processing of local defects.