Phosphate Pre-Treatment of Steel
Always Powder Coating Pty. Ltd.
| Always Powder Coating have replaced the
conventional iron phosphate pre-treatment system with
an organic process that operates at room temperature.
The savings from reduced energy, labour and waste
disposal costs amount to $60,000 per annum. |
Contact
- Mr Gavin Connelly
- Australian Chemicals and Coatings Pty. Ltd.
- 2 Lanyon Street
- Dandenong
- VICTORIA 3153
- Ph: 61 3 97525855
- Fax: 61 3 97525092
Background
Always Powder Coating (APC) operates a powder coating
facility in Melbourne's eastern suburbs and specialises in
powder coating of metal chair frames, bike frames and other
similar metal components. An important part of powder coating
is the cleaning and pre-treatment of the metal. Cleaning is
required to remove residual oil and dirt, and phosphate
pre-treatment is commonly used to enhance paint adhesion and
corrosion resistance.
The Process
The most common iron phosphating system uses a multi-stage
water-based process. In the case of small items, this involves
hanging the parts on racks or hooks suspended from an
overhead conveyor. The parts then pass through the
phosphating machine, where various solutions are sprayed over
the parts. The process usually consists of the following
distinct stages:
- alkaline degreaser;
- rinse;
- iron phosphating solution;
- rinse;
- passivating solution (optional, but usually chrome
based); and
- rinse.
The solutions are recirculated through storage tanks
located next to each stage. Some of the solutions require
heating to approximately 60 degrees Celsius. There is
significant sludge generation associated with the iron
phosphating solution, which necessitates frequent desludging
of the tank. In addition, all the solutions require periodic
dumping, make-up water is required to replace evaporation
losses, and there is a continuous overflow from the rinse
stages.
In some operations, solvent-based (usually
trichloroethylene) vapour degreasing systems replace the need
for an alkaline degreasing stage. Vapour losses from such
systems can be quite high, and even when totally enclosed
there will still be some vapour loss.
Cleaner Production Initiative
APC have installed a phosphate treatment system that uses
an organic phosphating process. The system uses a single
solution of ECOPHOR A/477 that acts to both degrease the
parts and provide a phosphate coating. The ECOPHOR solution
is based on an inorganic polyphosphate dissolved in a low
toxicity organic solvent medium. The solution has a very low
vapour pressure and is therefore suited to conventional spray
application.
The ECOPHOR solution works by initially degreasing the
work piece and then depositing a thin layer of iron
phosphate. A film forming polymer is deposited co-currently,
and this is chemically linked with the metal via the
phosphate. This polymer has the ability to absorb oils and
greases, thus allowing the solution to be used continuously,
without any requirement for periodic dumping. Degreasing of
the metal surface, with the aid of the organic solvent, is
followed by the adsorption of the oils and greases by the
polyphosphate. The polyphosphate polymer will absorb a
maximum of 6 per cent of its weight in oils and greases. This
represents an average oil loading of 1.5 grams per square
metre of treated metal surface. The final structure of the
organic polyphosphate surface comprises a very thin layer of
amorphous organic modified iron phosphate covered by a
polymeric film. This is in contrast to a conventional
phosphate process that produces a porous phosphate coating.
Heat curing is required to set the phosphate coating, as
it is this process that drives off the remaining solvent and
causes the free acid groups in the polymer to react with the
substrate to produce a three dimensional structure.
The ECOPHOR solution can be applied by either spray
(overhead conveyor) or dip techniques. Both systems have been
installed by APC. Small parts (chair frames, etc.) are
processed through a tunnel, which comprises a single ECOPHOR
application, followed by an air knife to remove excess
solution, and finally a curing oven. Large parts are treated
in a converted vapour degreasing tank, where the solution is
simply sprayed over the part, which is then oven cured. In
both systems the parts are sprayed with the ECOPHOR solution
for 60 seconds, followed by drying at 90-120 degrees Celsius
for between 4 - 6 minutes.
Although the spray system
equipment is similar in appearance to a conventional iron
phosphating tunnel there are a number of differences that are
critical to the successful operation of the system,
including:
- operating at reduced spray pressure;
- reducing the number of risers; and
- using stainless steel for all tanks.
The solutions are tested fortnightly for oil content,
resin content (polymer) and phosphoric acid. Additions are
made where required. This is in contrast to a conventional
phosphate process where rigorous process control testing is
required (at least once daily).
Advantages of the Process
The adoption of this process has eliminated the generation
of wastes normally associated with phosphate treatment
systems. This has led to considerable savings. The benefits
of the system to APC are:
- no trade waste discharge;
- no sludge generation and subsequent waste disposal
costs;
- no down time required for desludging of tanks;
- no water make-up to tanks;
- daily process control monitoring is not required; and
- solution heat-up is not required, which means that
the line can be started more quickly, with a
consequent saving in labour costs.
ECONOMIC BENEFITS
|
| Total Capital
Cost |
$177,800* |
| Annual Savings |
|
| Chemical Costs |
$6,000 |
| Waste Treatment |
$4,000 |
| Reduced Energy
Costs |
$20,000 |
| Labour Savings |
$30,000 |
| Total |
Annual Savings
$60,000 |
| Payback Period |
2.8 years |
* includes capital cost for tunnel and curing oven
This process is capable of treating a range of different
metals, including steel, aluminium and aluminium alloys, cast
iron and galvanised steel. Improved corrosion protection, in
comparison with conventional iron phosphating without a
chrome passivating stage, is achieved in salt spray tests
(200-250 hours versus 100 hours).
Cleaner Production Incentive
The significant issue in adopting this process was that
APC was reluctant to install a conventional phosphating
system due to the problems associated with:
- heating of solutions
- sludge generation
- the aggressive environment (steam and overspray of
chemical solutions) and its impact on conveyor wear -
APC had previous experience of a conveyor system
seizing up within three months of installation
Barriers
There was some initial scepticism about the ability of the
new process to provide a quality phosphate coating. This has
been overcome and APC is now coating components for a number
of high profile companies that have strict quality
requirements.
Another potential barrier to uptake of the ECOPHOR system
was the higher upfront chemical cost. The initial chemical
cost was approximately $8,500, compared to $500 for an iron
phosphate system. This high initial cost has, however, been
offset by the benefits, including lower annual chemical
costs, reduced energy usage, reduced trade waste generation
and elimination of sludge disposal.
Case Study Prepared: January 1997 by ACCP
The Cleaner Production Case Studies Directory is part of EnviroNET Australia.
- For more information contact:
Environment Australia
Environment Protection Group
PO Box E305
KINGSTON ACT 2604
AUSTRALIA
Email: cproduction@ea.gov.au
URL: http://www.environment.gov.au/net/environet.html