The corrosion takes place due to oxidation and galvanic actions.
The rust formed is generally represented by
Metal oxides are very common commodities, widely applied, and have many
different varieties. For example, zinc oxide sintered together with other
metal oxide additives have been made into
non linear resistors,
which are called Varistors for surge suppressing function. The suppressing
function has been applied for switching and for protection of random
voltage protections.
Iron oxide and other metal oxides are used in thermite reactions, and this has been applied in many ways, including welding in spaceship repairs. Iron oxides are also the raw material for all magnets and magnetic materials used for computer disks and recording tapes.
Not all metal oxide form a scale. In general, when the oxide formed is not very dense, it is not under stress, and the oxide layer forms a scale. Usually, a mole of metaloxide should occupy more volume than a mole of the metal itself. If this is true, the oxide is not under stress, and a protective scale is formed. In general, if the volume of the metal oxide per mole of metal is greater than the molar volume of the metal, the oxide will form a protective scale.
On the other hand, if the oxide formed occupies a smaller volume than the volume occupied by the metal itself, the oxide layer will be under tension and at some point it will crack. Thus, the oxide offers no protection for further oxidation.
The molar volume is easily calculated by dividing the molar mass by the density:
Example 1
Solution
The molar volumes are given below:
Discussion
Since the molar volume of the oxide is less than that of the metal, the
oxide does not form a protective scale.
Example 2
Solution
The volumes per mole of La are given below:
Discussion
It has been a well known fact that alumimun oxide forms a protective
scale. These data confirm the fact, and now you have an explanation for
crrosion. However, we should realize that sometimes the metal oxide does
not form a protective layer even if the oxide is not under tension.
The linear rule. When the oxide offers absolutely no protection, the progress of oxidation is a linear relationship with time. This has been calle the rectilinear rate law by Swaddle.
When the oxide layer gives some protection, the parabolic law pply. This law is formulated with the consideration,
When the oxide layer forms a protetive layer, but large flakes crack and leas to faster oxidation as a result. Then, the rate is a combination of the linear rule and the parabolic law.
When the oxide forms a good protective layer, the logarithmic rate law and the inverse logarithmic rate law) have been applied. Some suggested formulas are:
y = b ln (k t + 1)
The properties of metals and its oxide play roles in the rate of corrosion. There is no set rate laws, and every case must be studied carefully. So far, we have hinted a few has models for exploration and analysis of corroson problems.
Most people consider the oxidation of iron results in the formation of a film of iron sesquioxide, which is a term given to red iron(III) oxide, Fe2O3, which is also called ferric oxide. In reality, the oxidation and corrosion of iron is a very complicated process.
The corrosion takes place due to oxidation and galvanic actions.
The rust formed is generally represented by
The common iron oxides are
The basic structures of iron oxides and iron oxy hydroxides can be described as a close packing of oxygen (or hydroxide) with iron ions in the octahedral sites.
The structures of goetite and hematite may be describe as an approximate hcp close packing of O2- or HO- ions with some of the octahedral sites occupied by iron ions. Thus, these two types of structures are usually designated as a phases.
The structures of lepidocrocite and maghemite may be described as an approximate ccp (fcc) packing of O2- or HO- ions with some of the octahedral sites occupied by iron ions. Thus, these two types of structures are usually designated as g phases.
Some of the iron ions can be replaced by other metal ions, forming solid solutions in a process known as isomorphous substitution in terms of structural chemistry.
