An Overview of Phosphating
By Michael Marzano and Tony Oriti
Pavco, Inc.
Corrosion protection, paint adhesion, uniform coverage and cost effectiveness
are qualities that have seen sought by metal finishers from the infancy
of metallurgy and can all be furnished by the use of a phosphate coating.
Phosphating is a relatively simple process that has been used for well
over a century to protect metal from corrosion. In general, phosphating
is the conversion of a metal surface into an insoluble and integrated lattice
of metal and crystalline phosphate. This process takes place by the treatment
of the surface with a solution of phosphoric acid and other chemicals,
which react with the metal to form a slightly protective layer.
A Brief History
The phosphating process dates back to 1864 when de Bussy obtained a British
patent for the treatment of red-hot iron with a mixture of coal-dust and
calcium dihydrogen phosphate. This mixture added a corrosion-resistant
layer to the metal surface. Other scientists at this time also proposed
different treatments, including the immersion of the heated iron into a
solution of phosphoric acid and a mixture of sodium and ammonium dihydrogen
phosphates. In 1906, T.W. Coslett of Birmingham, England, obtained a U.K.
patent for a revolutionary idea involving the treatment of a properly cleaned
ferrous substrate with diluted phosphoric acid. This bath operated at temperatures
close to boiling, and so to reduce the effect of the intense chemical reaction,
Coslett introduced iron filings into the solution. These filings reacted
with the phosphoric acid to form ferrous dihydrogen phosphate. Coslett’s
most significant improvement, however, took place when he used zinc dihydrogen
phosphate added directly to the phosphoric acid. By doing this, he became
the originator of a process that remains in use to this day. These early
solutions, which operated at temperatures near boiling, have paved the
way for the more familiar phosphate baths of today that operate at temperatures
ranging from 100 to 180 degrees F and are used industry-wide.
During World War II, it was discovered that a light phosphate coating
could be used as a primary base to provide excellent adhesion for subsequent
paint films. In addition to paint, it was found that a heavy phosphate
coating had quite an affinity for oils or waxes, and the use of these over
the phosphate coating would supplement the corrosion protection of the
phosphate. Phosphating plus oil or wax is commonly used to treat cast,
forged and hot rolled steel parts. Because of their abilities as lubricant
foundations, these integrated phosphate and oil coatings are used to provide
resistance to wear, galling or scoring of moving parts. However, on the
basis of pounds of chemicals consumed, or tons of steel treated, the greatest
use of phosphate coatings remains as a base for paint.
Phosphating … The Process
The conversion of a metal surface to an insoluble phosphate coating provides
that surface with a physical barrier against moisture. The amount of corrosion
protection that a phosphate coating imparts to a metal surface depends
on the uniformity of the coating, as well as the thickness, density and
crystal size. Coatings can be produced with a wide range of thickness depending
on the method of cleaning before treatment, adjustment in the concentration
of the phosphating solution, and duration of treatment. In phosphating,
no electric current is used, and formation of the coating depends primarily
on contact between the phosphating solution and metal surface and on the
temperature of the solution. Consequently, uniform coatings can be produced
on irregularly shaped articles in recessed areas and on threaded parts,
as well as on flat surfaces. Being a conversion coating, phosphating can
be accomplished by immersion or spraying. Small parts with many recessed
areas, such as nuts, bolts and other small stampings are immersion coated
in a tumbling barrel. Parts that are too large for this barrel process
may be immersion coated by the racking method. If the part is still too
large for rack application and has very flat surfaces, it may be treated
while on a conveyor system by a sprayed phosphate solution. Phosphate coatings
can have a thickness ranging from less than 0.1 mil to more than 2.0 mil.
Typically, though, the amount of coating is measured by milligrams of coating
per square foot, or grams per square meter, rather than by thickness in
thousandths of inches. Of the many phosphates proposed, only iron phosphate,
zinc phosphate, manganese phosphate, zinc-manganese phosphate, zinc iron
(II) phosphate, mnganese-iron (II) phosphate, and zinc-calcium phosphate
are of industrial importance. Although aluminum and magnesium can be treated
through this process, ferrous and zinc surfaces remain the most commonly
phosphate-treated metals in the industry. The three most commonly used
phosphate coatings are zinc phosphate, iron phosphate and manganese phosphate.
Iron phosphating was the first processes to be used commercially and originally
produced a dark gray coating with coarse crystals.
Unlike early iron phosphating, the solutions used today produce an amorphous
coating of exceedingly fine crystals with an iridescent blue to bluish-brown
color. Since iron phosphate crystals are translucent, their color is modified
by the surface on which they are deposited. While iron phosphate coatings
can be applied to steel to provide a receptive surface for the bonding
of fabric, wood and other materials, their chief application remains as
a foundation for paint. Under appropriate processing conditions, iron phosphate
coatings have excellent adherence and show good resistance to flaking from
impact or flexing. Spray application of iron phosphate is most commonly
used, although immersion applications are also practical. The typical range
of coating weight is 20-80 mg/ft2 (0.21-0.86 g/m2). Exceeding this thickness
hasn’t shown to provide a significant amount of benefit, while underachieving
this thickness tends to show a non-uniform, or discontinuous, coating.
The zinc phosphate coating, which can be attributed to Coslett’s use
of zinc dihydrogen phosphate, possesses a wide range of weights and
crystal characteristics. These coatings can be heavy films with coarse
crystals or ultra-thin microcrystalline deposits. Depending primarily on
the carbon content of the underlying steel, they may vary from light to
dark gray in appearance (darker as the carbon content increases). Microcrystalline
coatings usually are darker gray than coatings of the same weight and have
coarser crystals. Zinc phosphate coatings can be applied by spray or imersion
and may be used as a paint, wax, or oil base; an aid to cold forming or
rust-proofing; and for increasing wear resistance of moving parts. The
weight of spray zinc phosphate coatings over steel can range from 100 to
1000 mg/ft2 (1.1 - 10.8g/m2), while immersion coatings can range from 150
to 4000 mg/ft2 (1.6 - 43g/m2).
The use of manganese phosphate coatings can be traced back to R.G. Richards
and his system patent that was published in 1911. Richards, like Coslett,
used a solution of phosphoric acid, but instead of zinc, manganese dihydrogen
phosphate was added directly into the bath. This type of process is also
applied primarily to ferrous parts, most importantly on internal combustion
engine parts where the phosphate coating acts as a lubricant carrier to
prevent galling. Manganese phosphate coatings are usually black or dark
brown, depending on the amount of manganese dioxide included in the
coating. Because almost all manganese phosphate coatings are used as an
oil base, and oil intensifies the black coloring, manganese phosphate coatings
usually appear to be black. This type of phosphate is only applied by immersion
and requires immersion times of five to 30 minutes. Coating weights are
typically 500 to 3000mg/ft2 (5.4 – 32.3 g/m2), but can be heavier if needed.
Usually, the preferred manganese phosphate coating is tight and fine grained
rather than loose and coarse grained. Generally, a manganese phosphate
crystal is softer and will break down more readily than a crystal of zinc
phosphate. Manganese phosphate plus oil or wax is also used on cast iron
and steel parts. Although manganese phosphate generally costs more to apply
than zinc phosphate, the greater thickness of the coating encourages retention
of more oil or wax, and thus may provide greater resistance to corrosion.
The mechanism of all phosphate coatings takes place in an acid bath
that contains the coating chemicals. The chemicals react with the metal
to be coated, and at the interface, a thin film of the solution is neutralized
by reaction with the metal. When this solution becomes neutralized at the
interface, the solubility of the metal phosphate is reduced and a precipitate
is formed as a crystal. These crystals are attracted to the surface of
the metal by the normal electrostatic potential within the metal.
Even though all phosphate baths are acidic in nature and attack the
metal being coated, hydrogen embrittlement seldom occurs as a result of
the phosphating process. This is primarily because all phosphate baths
contain depolarizers or oxidizers that react with the hydrogen as it is
formed and render it harmless to the metal. Typically, zinc and manganese
phosphate baths use these depolarizers as accelerators. This can be a mild
oxidant, such as a nitrate, or a more vigorous nitrite like chlorate or
peroxide. The purpose of these accelerators is to speed up the rate of
the coating and to reduce the crystal size. This is accomplished by the
ability of the accelerators to oxidize the hydrogen from the surface of
the metal being coated. The phosphate solution can then contact the metal
continuously, permitting completeness of reaction and uniformity of the
coverage. Accelerators also have an oxidizing effect on the dissolved iron
in the bath, thus extending the useful life of the solution. Although some
iron phosphate baths do not require accelerators, many still incorporate
oxidizing agents to accelerate the phosphating process.
The pH of the phosphate bath depends on the type of phosphate compound
and its method of application. Manganese and immersion zinc phosphate baths
operate in the pH range of 1.8 to 2.4, whereas spray zinc phosphate solutions
can operate at a pH as high a 3.0. Iron phosphate baths can operate at
a pH of 3.0 to 6.0.
Since 1864, phosphates have proven to be an inexpensive and widespread
method of conversion coating metal surfaces. Phosphates have shown value
in their ability to be used to improve corrosion resistance, prepare surfaces
for metal finishing, and, most importantly, condition surfaces for painting.
With this versatility, the future of phosphates looks very healthy.
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