SurTec
Technical Letter 10
Dephosphating of Parts before
Heat Treatment
Using the Example of Screw Production
Reiner Grün April 2002
Revised February 2012
Before reshaping steel wire, e.g. forming the thread of screws, it has to be
phosphated. The phosphate layer allows a better sliding and gives a
temporary corrosion protection. These phosphate layers have to be
removed completely before tempering. Otherwise glass like and diffusion
inhibiting iron phosphorases would be formed during the heat treatment.
These phosphorases are very brittle and would result in blisters and faults
whilst successive zinc plating.
By specific bath maintenance and good monitoring of the sequence of
treatment steps, dephosphating baths can be increased both, life time and
efficiency. This Technical Letter advices procedures proven in praxis for an
increasing service life.
Introduction
Iron wire as base material for the production of screws passes several steps like forming,
drawing for reducing the diameter or rolling of threads. For better properties and for
temporary corrosion protection, the wire has to be phosphated (zinc phosphating) and
treated with support compounds, such as draw-aids and/or water-insoluble soaps.
The phosphate layer and the support compounds have to be removed completely before
the following heat treatment (glowing, - possibly tempering - , quenching and annealing). If
not, crack products of the support compounds will be formed as well as glass like iron
phosphate layers.
These residues can behave diffusion inhibiting at thermochemical diffusion processes like
nitrifying. At following galvanic processes (e.g. zinc plating) they can lead to blisters or
faults, since they can hardly be removed during the pretreatment, by pickling over a long
period. However, strong and long pickling leads to a high diffusion rate of hydrogen into
the bulk with all its disadvantages (see SurTec Technical Letter 8). Thus, the time for
pickling should be as short as possible.
Similar problems will occur, if the oil of the quenching process is not removed completely.
The dephosphating is done in a highly alkaline solution (pH-value > 11). Yet, the applied
support compounds cause problems.
Since the organics containing chloride being banned (disposal, AOX-problem) from
production, the chlorinated paraffines (draw supporters) have been replaced by highly
ester-containing compounds. Esters improve the tribological properties of supporting
compounds indeed.
Esters are synthesized by reaction of carbonic acids and alcohols. Also natural oils
(triglycerides) are esters (native esters, e.g. rape or root oil) composed of glyceride and
long chained fatty acids (carbonic acids). Via hydrolysis (saponification) in aqueous
alkaline solutions, these esters are separated into soaps and alcohols. This process of
saponification is accelerated by temperature and mixing.
The crack product soap usually behaves emulsifying and prevents oil to demulsify in the
cleaning bath. Furthermore, the water soluble soaps generate and stabilize foam and are
causing problems. Additionally, these soaps can react with magnesium or calcium diluted
in the water and with iron or zinc ions from parts forming insoluble chalk or metal soaps,
which can precipitate on the parts and on the process equipment.
In the following, hints are given for bath care and examples for the process sequences. It
will be shown how the cleaning baths can work more efficient with an extended service
life. Even a possibility of a waste water free process is described.
SurTec Technical Letter 10 –Dephosphating of Parts before Heat Treatment page 2 of 5
1. Pre-Cleaning
In order to reduce the saponification to a minimum, prior to the dephosphating bath a low
alkaline (pH < 9), demulsifying cleaning with successive rinsing should be applied, for
instance SurTec 131 (builder) + SurTec 089 (surfactant for soak application) or SurTec
086 (surfactant for spray application, foam reduced). For easier or automatic dosage (by
conductivity), both compounds are liquid.
Even in the weak alkaline medium a few soaps will be formed, however, they can be
destroyed (and removed) by adding a demulsifier (e.g. SurTec 930). The demulsified oil
and the small amounts of mud (zinc phosphate, metal soaps, chalk soaps), can be
removed in an oil separator, installed in a bypass. The oil separator has to be provided
with a sloping bottom and an outflow to let off the deposited mud. Alternatively, a slope
clarifier can be used. The demulsified oil has to be removed continuously to prevent
further saponification.
Due to the crystalline structure of the zinc phosphate layer, the supporting compounds
cannot be removed to 100 %. During pre-cleaning, the quenching oils can also be
removed.
2. Dephosphating
For the removal of the phosphate layer, highly alkaline (pH > 12), surfactant-free,
complexing agent containing products are used, for instance SurTec 198 or SurTec 199 or
SurTec 194 (alkaline compound) + SurTec 419 (complexing agent). The complexing
agents are biodegradable and the metal complexes (zinc, iron) can be splitted off easily in
usual waste water treatment.
For easier or automatic dosage (by conductivity), both compounds are liquid.
If a pre-cleaning is not possible (e.g. not existent in the present process line) surfactants
have to be added to the dephosphating bath (e.g. SurTec 089, SurTec 086). The soaps
being formed can be destroyed completely by using demulsifying agents like SurTec 930.
Depending on the thickness of the phosphate layer and on the throughput, large amounts
of mut can be built up. They can be removed by slope clarification, separators or
decanters. Demulsified oil can be removed by an additional oil separator. Again, the
demulsified oil has to be removed continuously to prevent further saponification.
3. Rinsing and Corrosion Protection
In the successive rinses, water soluble corrosion inhibitors as SurTec 534 or SurTec 533
can be added. Both products prevent corrosion while storing. They evaporate at about
180-200°C and do not disturb the following heat treatment or thermochemical diffusion
processes, respectively.
SurTec Technical Letter 10 –Dephosphating of Parts before Heat Treatment page 3 of 5
4. Examples for the Processes Sequence
Pre-cleaning:
concentration temperature
SurTec
[%vol] [°C]
131 + 089 3-4 + 0.5 50-70
Dephosphating in succession of pre-cleaning:
concentration temperature
SurTec
[%vol] [°C]
198 or 199 5-10 50-70
alkaline compound and complexing agent separately:
concentration temperature
SurTec
[%vol] [°C]
194 + 419 5-10 + 2-4 50-70
Dephosphating without pre-cleaning:
concentration temperature
SurTec
[%vol] [°C]
198 or 199 5-10 + 0.5 50-70
+ 089
alkaline compound and complexing agent separately:
concentration temperature
SurTec
[%vol] [°C]
194 + 419 5-10 + 2-4 50-70
+ 089 + 0.5
Rinsing, corrosion protection:
concentration temperature
SurTec
[%vol] [°C]
533 or 534 1-2 50-70
Drying
SurTec Technical Letter 10 –Dephosphating of Parts before Heat Treatment page 4 of 5
5. Example of a Waste Water free Process Line
For the process technique, different types of processes can be applied:
chain, barrel, rommel or snail processes.
A waste water free process could be established as follows:
SurTec Technical Letter 10 –Dephosphating of Parts before Heat Treatment page 5 of 5
