Phosphating of metals is one of the most important of surface
treatment methods and a number of modern metal finishing procedures
would not be possible without it. The main areas of application of
Corrosion protection in conjunction with organic coatings, e.g.,
paints and polymer films,
Facilitation of cold-forming processes, e.g.,
wire drawing and
Corrosion protection in conjunction with oils and waxes,
Corrosion protection with no subsequent treatment.
Phosphating can be executed using a variety of sequential processes.
The list of these and whether they are in fact called for or not,
depends on many factors of which the most important are;
Nature of the metal to be coated,
Shape and surface condition of the metal,
Number of parts and uniformity of their surface state,
Anticipated service duty of the phosphate coating,
Organisational factors, e.g., limit on number of processing lines,
The phosphating sequence is normally broken down into the following
There is normally a rinse between the individual processing steps in
order to remove the solution layer from the metal surface and to
avoid carry-over of this as an impurity to the subsequent stage.
According to circumstances, the processing sequence may be longer or
shorter. Thus, for example, pickling can be omitted if no corrosion
products are present on the metal surface. Then, too, some of the
individual stages can be combined, for example, cleaning and
Of the many phosphating systems which have been proposed, the following are of industrial importance:
manganese phosphate. Such phosphating systems are predominantly applied to iron and steel, as well as zinc surfaces whereas other metals which can be phosphated, e.g., aluminium, magnesium, are less commonly processed by this method.
Phosphate coatings which adhere well to the basis metal and which,
so far as possible, cover it completely, can only be formed on clean surfaces which are free from rust, scale,
soot and other corrosion products. Oil and grease films likewise inhibit phosphate coating formation, except when they are so thin that they are removed by initial etching stage.
of the pretreatment steps used before the phosphating itself, e.g.,
in the cleaning, can considerably affect the growth rate of the
phosphate coating, its thickness and the crystal size. These effects
can be chemical or mechanical and sometimes both together.
of metal pretreatment on coating weight depends on the phosphating
temperature. At high bath temperatures, the phosphate crystals form
on the blasted steel are largely of uniform size and very fine. On
samples which have only been degreased, very large and very small
crystals co-exists, whereas on blasted steel, there is much more
uniform coverage of very small crystals.
pretreatment of the metal, too, can significantly affect the
formation of the phosphate coating. These observations have
industrial significance in terms of alkaline degreasing and solvent
degreasing as well as acid pickling. Phosphating solutions based on
zinc, iron and manganese produce rougher phosphate coatings with
higher weight per unit area, when strong alkaline cleaners are used,
in contrast to solvent degreasing. In addition, longer phosphating
times are required for complete coverage of the metal.
that a large range of steels are capable of being phosphated is not
to imply that, in all cases, these are suitable for industrial
applications. Thus it has been found that deep-drawn steel, as
widely used in automobile industry, can be phosphated, painted and
subjected to accelerated corrosion testing.
materials industrially used, including electrogalvanized and hot-dip
galvanized steels, as well as zinc with small additions of copper or
titanium, used, for example, in diecastings, can be phosphated
without problems. To improve corrosion resistance and "white rust"
chromate passivation films must first be removed.
of the phosphate coatings lies between light grey and pitch black.
The lightest are the zinc phosphate coatings on zinc which, when
this is the only metal in the bath, contain no other cationic
species. When iron (II) also is present, and especially zinc and
nickel, darker, usually, mid-grey coatings are formed. Whit steel
substrates zinc phosphate coatings are darker than on zinc
substrates, which is explained in terms of the larger amount of iron
in the coating. Zinc-calcium baths produce coatings of similar hues
to the zinc-only baths. Very dark, anthracite-black coatings result
from the phosphating of steel in manganese-based solutions.