|
Spray equipment
Transfer efficiency
pneumatic high pressure spray gun
50 %
pneumatic low pressure spray gun
60 %
airless spray gun
70 %
pneumatic gun with electrostatic support
80 %
electrostatic bell application
90 %
Reine ESTA-Applikation (pure electrostatic application (disc))
95 %
Application processes for automotive coatings
46
3.4 Pre-treatment and primers
3.4.1 Substrates and corrosion
The most important task of automotive primers is protecting against corrosion. Even today, steel is
the preferred material for car bodies, although the quantity of plastic parts in cars is continually
rising. Other metals are used besides steel, with aluminium and magnesium being suitable for
special automotive parts. Positive properties of steel are
[45]
:
•
Steel is relatively inexpensive
•
Steel is easy to handle (to roll, cut, bend, and weld)
•
Steel has excellent application properties (elasticity, tensile strength, hardness)
•
Steel is relatively easy to recycle (to shred, scrap for blast furnaces)
In the past, steel was readily available, but now it is in short supply due to heavy demand from
the Far East.
As already described in the chapter on the historical development of automotive coatings, the first
cars were built along the lines of horse-drawn carriages. They had a chassis as a frame for the car
body. The first car bodies consisted of wood, and then of steel panels. Eventually, the first so-called
self-supporting car bodies were developed. The first car body of that type was constructed in 1922
[46]
. The first mass-produced car to have a self-supporting steel body appeared in 1935
[47]
. Now,
nearly all passenger cars are constructed that way. For adequate stability, some parts of the body
have folds or ridges and the outer shell is very compact. The various parts are welded together. But
there are also alternative joining processes, which are mainly used for new construction materi-
als. The most important goal of car body construction is to optimise structural stability (rigidity,
crash safety) while reducing the weight (to save energy and fuel). To this end, special grades of
steel have been developed that offer a combination of optimum shaping and optimum mechanical
stability (e.g. IF-steel, DB-steel).
The most important disadvantage of using iron and steel is corrosion. Iron corrosion has been
known since the Iron Age. There are steel grades that do not corrode, but they cannot be used
for car-body steel, as they do not have the shaping properties and the required stability. Conse-
quently, the new steel grades also have to be protected against corrosion.
All metals in contact with air acquire a layer of oxide on the surface after a while. The other
components of air (moisture and carbon dioxide) can transform the oxides into hydroxides and
carbonates. Some metals form dense, stable oxide layers, whereupon corrosion ceases. Unfortu-
nately, iron and most grades of steel react with oxygen, moisture, and carbon dioxide to form oxide
layers, which have totally different crystal structures (crystal lattice) than the metal. Thus, there
is no dense oxide layer and so corrosion continues until nearly all the metal is transformed into
oxidation products. The chemical process by which iron and steel corrode is clearly described by
the electro chemical local element model
[48]
. The underlying reactions that trigger corrosion are
illustrated in the local element model in Figure 3.4.1.
The key actor in the model is water, which supports all electrolytic processes. According to the
model, metallic iron is oxidised to the iron(II) cation, which goes into solution. The electrons
released by the oxidation reaction can reduce water and oxygen to hydroxyl ions (see Equation
3.4.1).
Equation 3.4.1
Automotive OEM coatings
47
The iron(II) ions react with hydroxyl ions and further oxygen to yield a precipitate of iron(III)
hydroxide. Iron(III) hydroxide can cleave water to form iron oxide hydrate or water-rich iron oxide
and may also react with carbon dioxide to yield carbonates (see Equation 3.4.2). This mixture of
the various products is commonly called rust.
Equation 3.4.2
The described process continues until most of the iron metal has been transformed into the reac-
tion products. It is therefore of vital economic importance that corrosion is avoided and that iron
and steel plant, equipment, vehicles and other objects are protected against corrosion.
Different measures can be taken to avoid corrosion of iron and steel
[49]
. One is to cover the iron or
steel surface with a metal that oxidises more readily than iron. Such metals are more reactive, as
defined in the electro chemical series of elements. In local electro chemical elements, that metal
– instead of iron – acts as the anode and is oxidised to cations, which go into aqueous solution.
If the metal takes the place of the iron, it is called a sacrificial anode, in which case the iron acts
as the cathode. This process is called active cathodic corrosion protection. The most common
sacrificial metal covering the iron is zinc. Zinc has the advantage that the oxide layers of zinc
form a homogeneous crystal lattice with those of the metal, and also adhere very strongly. The
corrosion process is stopped or significantly reduced if a thin layer of oxide is formed. Most car
bodies nowadays are made from steel with a zinc coating. The zinc layer is applied by a thermal
process (hot dipping) or, more commonly, by electro chemical reaction (galvanized steel)
[50]
.
Another method is to cover the iron with a metal that is less reactive in the electro chemical series
of elements. The most important metal used in the manufacture of objects for industrial use is tin.
Metal sheeting coated with tin is called tinplate. It is used mainly for cans and containers, but
also for some other objects. Many other iron or steel objects are covered with chromium or nickel.
Tin, chromium and nickel react with atmospheric oxygen to form thin, homogeneous, dense oxide
layers that escape further corrosion. Thus, as long as these metal layers are not damaged, the
underlying steel is protected against corrosion. Basically, the metal coating on the iron presents a
barrier to the corrosive agents. This method is called passive cathodic corrosion protection.
Similar methods are pre-treatment of the iron or steel and coating with a primer, or a combina-
tion of both. The outcome is layers which are intended to offer passive corrosion protection and
additionally present a barrier to corrosive agents. Iron and steel are not the only surfaces to be
protected by the aforementioned methods. Zinc, aluminium, and magnesium are also pre-treated
and primed. Since all these metals are used for car bodies, primers need to be developed which
offer optimum compatibility (adhesion, protection efficiency) with different metals.
Figure 3.4.1: Local element model for describing the corrosion of iron
Pre-treatment and primers
48
Do'stlaringiz bilan baham: |
