Сorrosion protection of aluminum

Pitting corrosion of aluminium

The types of aluminium corrosion

Aluminum may be subjected to various types of corrosion, which more or less visible to the naked eye. The main types of corrosion of aluminum and aluminum are:

  • general corrosion,
  • pitting corrosion,
  • crevice corrosion,
  • stress corrosion,
  • intergranular corrosion.

The most common type of corrosion

Pitting corrosion of aluminum, which, although it is the most common, but it is still up to the end unexplored. This type of corrosion is sometimes referred to as peptic corrosion and pitting. Below is a summary of the description of pitting corrosion of aluminum in the major work of the famous French expert Christian Vargel “Corrosion of aluminum” [1].

Figure 1 – Pitting corrosion of aluminium exposed to chloride ions [2]

Pitting corrosion - a localized form of corrosion, which is characterized by the formation on the surface of individual pits (ulcers) of irregular shape. Their diameter and depth depends on several parameters, are associated with the chemical composition and purity metal, corrosive environments and operating conditions.

Aluminum is apt to pitting in environments with a pH, which is close to neutral. This includes the conditions of the natural environment, such as, Freshwater, sea ​​water and humid air.

Unlike other metal corrosion of aluminum always well visible because of corrosion jazvochek, which are covered with white, bulky and jelly-like "pimples" of aluminum hydroxide Al (OH)3.

Pitting occurs, when the metal is placed in continuous or intermittent contact with an aqueous medium: fresh water, sea ​​water and moist air. experience shows, if that happens pitting, it always occurs within a few weeks of being in the water a corrosive environment.

for instance, you can imagine the disappointment yachtsman, when it detects an aluminum housing for its remarkable yacht surface pitting after only a few weeks after launching it on the water.

Pitting often confuses owners of aluminum products and equipment. Little of, she sometimes baffled by corrosion experts, which should explain this complex phenomenon and make lifetime predictions.

Pitting corrosion is really a complex phenomenon. Even today, its mechanism is not fully understood, despite the very large number of studies and publications on this topic in the past 90 s-plus years. From the very beginning of the active use of aluminum [1]!

It is well known, so this:

  • conditions for the initiation of pitting corrosion;
  • slowing pitting up to its full stop.

Therefore, at present most aluminum equipment morally obsolete, than the time to collapse pitting. Even in wet conditions, aluminum products will serve for decades.

Pitting corrosion process

Aluminum refers to metals, which have on the surface of the passive oxide film. Such metals are prone to local corrosion in local damage of the passive film. This leads to the formation of corrosion pits, that under the right conditions for it can develop and grow. Pitting corrosion of aluminum shows two distinct stages:

  • birth and
  • height.

Nucleation

It has long been known, that pitting corrosion develops in the presence of chlorides as follows:

  • absorption natural oxide film of aluminum chloride ion Cl;
  • film break in weak points;
  • the formation of microcracks several nanometers wide;
  • the emergence of corrosion pits.

In a short time, can form a large number of microscopic corrosion pits - to 10 million per square centimeter. The density of the pits depends on the type of aluminum alloy:

  • 10 thousand per square centimeter for pure aluminum, comprising 0,1 % impurities;
  • 10 billion per square centimeter for the alloy, comprising 4 % copper.

However, most of these microscopic corrosion pit stop in the growth of a few days.

Deeping

Only a very small fraction of the nucleated corrosion pits will continue to grow as a result of electrochemical reactions at the anode and cathode (Figure 1) [1].

Figure 2 – The mechanism of aluminium pitting corrosion [1].

The anode is a corrosion pit bottom, and cathode - aluminum surface around the fovea. As a result of electrochemical reactions at the anode corrosion "digs" hole deeper into the formation by aluminum ions Al3+. These ions diffuse towards the exit from the fovea, where they meet with an alkaline medium in the form of OH, as a result of which aluminum hydroxide Al (OH) is released at the cathode3.

The complete corrosion reaction of pitting corrosion of aluminum has the form [1]:

2Al + 3H2O +3/2 O2 → 2Al(OH)3.

The accumulation of corrosion products forms above the corrosion pit formation, similar to the dome of the volcano, progressive blocking entry therein. This complicates the exchange of ions between the bottom surface of dimples and Aluminum, special, if this involves chloride ions. This explains why, why pitting eventually slows down or even stops completely.

The rate ​​of pitting corrosion

experience shows, that in most cases the rate of corrosion pits deepen in vivo, such as fresh water, sea ​​water and rainwater decreases with time. This explains the very long service life (several decades) of aluminum., which was used in building structures (roofing sheets), in ship constructions etc..

Numerous experiments have shown, that the rate of deepening of corrosion pits in aluminum alloys slows down in proportion to the cube root of time:

d = kt1/3

Where

d is the depth of the corrosion pit;
t - time and
k - constant, depending on the alloy and operating conditions.

From this formula it can be seen, that an increase in, for example, aluminum pipe wall thickness for supplying liquid or gas to twice its service life is increased eightfold.

Characteristics of pitting corrosion

Unlike general corrosion pitting intensity and speed can not be estimated nor loss of mass determination, or determining the amount of hydrogen evolution. Really, these measurements make little sense. The thing is, that one very deep and single fossa can only give a very small loss of mass On the other hand, a large number of small surface pits can lead to a large loss of mass.

Therefore, pitting corrosion is evaluated according to three criteria:

  • density, that is, the number of corrosion pits per unit area;
  • the rate of deepening of corrosion pits;
  • the likelihood of pitting corrosion.

Pit density

density measurement does not present any particular difficulties. It consists in simply counting the number of holes, which are visible on a predetermined area and length of the product or a sample. experience shows, what 1 square decimeter flat rolled and 1 decimeter tube are sufficient to obtain reliable information about pitting density.

If the number of pits is small and they are scattered over the surface, the depth of their generally greater, than the pits, which in large quantities are evenly dispersed on the surface.

The rate of pit deepening

This factor is the most important. The growth rate of the depth of corrosion pits is much more important, than their density, since the life of aluminum products depends precisely on, how to grow rapidly in the depth of corrosion pit. Note, that the depth of corrosion pits is independent of the thickness of the metal.

The depth of corrosion pits measured at the end of the test period or a predetermined period of life. At a given surface area, for example, 1 dm2, find 5 or 10 the deepest pit and measure their depth. This is typically done using a microscope with a sufficient increase in. To process measurement results, special statistical methods are used [1].

The figure 2 shows the maximum pitting corrosion depth in the test samples in a marine atmosphere of aluminum alloys 1050, 3003 и 5052. As seen in the growth rate of corrosion pit depth is proportional to the cube root of time.

Figure 3 – The deep of pitting corrosion of 1050A, 3003 and 5052 aluminium alloys in sea atmosphere [1].

The probability of pitting corrosion

The probability of pitting corrosion is determined experimentally. To do this requires a large number of samples. The probability of pitting corrosion (in percentage) is determined by the formula

p = 100 · (Np/N)

Where

p is the probability of pitting corrosion;
Np - the number of samples, exposed to pitting;
N - total number of samples tested.

Probability pitting growth rate and depth of corrosion pits are two factors, which characterize the durability of aluminum in water. They are not related to each other: this aluminum alloy can have a high probability of pitting corrosion and a low rate of growth of the depth of corrosion pits.. The opposite situation is also possible: low probability of pitting corrosion, but the high rate of growth of the depth of corrosion pits. Of course, first embodiment is more preferable.

Resistance of alloys to pitting corrosion

  • All aluminum alloys are somehow prone to pitting corrosion in vivo. experience shows, that the resistance to pitting corrosion above, if the density of holes is high incipient.
  • For thin products with thickness below 100 micrometers depth of pitting corrosion can be limited by introducing an aluminum alloy 1 % gland. Examples include alloys and 8011A 8079. Increasing the number of intermetallic compounds Al cathode3Fe increases the amount of incipient corrosion pits.
  • For the same reason, a small amount of copper is added (0.10-0.20 %) to the alloy 3003, to increase the number of places of origin of the corrosion pits on the Al intermetallic particles2Cu.

Sources:

  1. Corrosion of Aluminium / Christian Dragline - Elsevier, 2004
  2. TALAT 1252