FIELD OF INVENTION:
The present invention describes the use of waste pickling liquor as an ingredient
to formulate phosphate solution for steel substrate and the process thereof for
application of this solution on the surface of rebar, wire, tube and steel sheet to
get an excellent phosphate coating. The phosphate coating is used to improve
resistance against early red rust appearance as well as to improve good bonding
with concrete for rebar and good anchoring with paint for sheet metal.
This coating also provides excellent resistance capability against chloride attack.
BACKGROUND OF THE INVENTION:
This invention relates to a process of forming a zinc-containing phosphate
conversion coating layer on an active metal surface. Chromate treated steel
sheet is not favoured due to the possibility of hexavalent chromium being
contained therein, and hence the demands on phosphate treatment have
increased.
The drawbacks associated with the application of a zinc phosphate coating
solution include additional process steps which are required in its application.
The additional process steps include pickling, rinsing and heating the surface
before and after application of the zinc phosphate solution. The rinsing and
heating processes may require a considerable amount of time.

The prior art includes solutions and methods for reducing the additional steps associated with the phosphating process and the costs associated with these processes. For example, U.S. Pat. No. 4,659,395 by Sugama et al., provides a process for applying a zinc phosphating solution which does not require subsequent rinsing. However, the prior art of Sugama et al. does require a heating process to dry the zinc phosphating solution from the surface being treated.
U.S. Pat. No. 6,117,251 is directed to the use of polyvinyl alcohol, along with zinc oxide, phosphoric acid, and a metal salt in an aqueous solution. Although this composition offers advantages, it has been found that, in some situations, the presence of polyvinyl alcohol tends to cause foaming, solubility problems, and causes the coating to be too sticky.
Waste pickling liquor (WPL) is generated when defective galvanised material is treated in acid solution to remove defective coating from steel surface at tube and wire galvanising plant. The zinc and iron content in WPL are 100 and 15 g/L respectively when the solution is discarded. This high level of zinc and iron are being wasted due to non-availability of technology. Zinc and iron both elements are very useful for formation of phosphate compounds. The goal of the present invention is to develop a phosphate coating solutions on steel substrate using WPL which will provide resistance against early appearance and good anchoring with paint & concrete structure.

OBJECTS OF THE INVENTION:
In view of such situation, the object of the present invention is to develop temporary corrosion resistant coating solution using waste pickling liquor to avoid early discoloration of steel surfaces during transportation and storage. Another objective of the present invention is to develop the passivation system without separate pickling arrangement.
In the present invention a phosphate coating solution for steel substrate was developed using WPL.
DETAILED DESCRIPTION OF THE INVENTION:
The materials used in the present invention are TMT rebar, wire, tube and steel sheet. Due to the electro chemical reaction the red rust is formed on the rebar surface. Although, it is not very detrimental from corrosion point of view, but aesthetically it may not be acceptable to the customer. Hence, some post treatment of the steel surface is required to prevent this early rusting process. The conventional nitrite or chromate coating is not sufficient to avoid early discoloration during transportation and storage. Therefore, the present invention provides a phosphate solution which provides the coating on the steel surface without separate pickling operation.

The basic ingredients and characteristics of WPL are shown in Table 1. Table 1. Composition of waste pickling liquor

Concentrated phosphoric acid was added in the WPL solution and pH of the solution dropped down to 0.9 after addition of phosphoric acid. Then sodium hydroxide solution was added to increase the solution pH up to 5. White precipitates were allowed to form and settle down at the bottom immediately after pH reached 5.
The precipitates were separated out and used for subsequent characterisation. XRD analysis confirmed precipitates are phosphate compounds Powder X-ray diffraction was conducted of the precipitate. Phosphate compounds are hoepite, phosphophylite and spencerite. Precipitates thus obtained are added in water and dissolved subsequently by drop down pH of the solution. Phosphoric acid is used to drop the solution pH. After this step, solution is heated at around 70 degree C for surface passivation of steel samples. Steel surface is reactive to this solution and Fe is dissolved in the solution. As a consequence pH is increased at the steel surface which is favorable condition for nucleation of phosphate crystal and deposition on the steel surface. Phosphate coating on the steel surface

formed by autocatalytic reaction. These precipitates were used for phosphate treatment on steel substrate under condition as mentioned below (table 2).

Table 2. Process conditions for phosphate coating on steel substrate
The phosphate coating as per the current invention contains the following
ingredients (table 3):
Table 3 Phosphate coating composition:

Zn & Fe are being used as metallic components in the phosphate coating. Zn is active metal compared to iron and hence expected to provide very good sacrificial protection. Further, Phosphorous is very good in corrosion resistance and anchoring with paint or concrete structure. Treatment can be done on steel surface free from any oxide scale at the top (WOS) as well as containing thin high temperature oxide scale at the top (OS). Fe gets dissolved from WOS steel surface and local rise of pH favours for formation of phosphate coating on the

steel substrate whereas Fe2O3 gets dissolved from the steel surface containing high temperature oxide scale at the top and local rise of pH favours the formation of phosphate coating on the steel substrate. Following reactions happen when oxide scale containing steel surfaces are treated.

It is evident from Fig. 1 that thicker (7 µm) phosphate coating was obtained on
the oxide containing steel substrate.
Pickling operation is done if required to remove oxide scale before phosphate
treatment. In the present work, we first optimized the process parameters to
remove high temperature oxide scale from steel surface. Pickling process
parameters were as mentioned in table 4.
Table 4 Pickling process conditions to remove oxide scale from steel surface


After pickling, rinsing is done thoroughly in tap water before dipping all the samples in an phosphate solution. Following reactions happen as mentioned below when oxide scale free steel surfaces are treated.

It is evident from Fig. 2 that comparatively thinner (4-5 µm) phosphate coating
was obtained on the oxide scale containing steel substrate.
Table 5. Time for first appearance of red rust of phosphate coatings in different environments:

As evident from the table above, the composite coated material showed significant improvement in corrosion resistance against chloride/open atmosphere compared to bare steel. The surface roughness of the coatings is shown in Table 6. Table 6: Roughness of the uncoated and phosphate coated steel substrates


Phosphate coated steel surface shows more Ra values than the steel surface without phosphate coating. Phosphate coated steel substrate also improved up to 30% increase in bonding with concrete structure as per standard of IS 1786(1985). This standard indicates change in bond strength of phosphate coated bar compared to a plain bar in concrete structure

WE CLAIM:
1. A process for developing temporary corrosion resistant coating for a
metallic substrate, the process comprising:
adding concentrated phosphoric acid to waste pickling liquid solution;
adding sodium hydroxide solution to increase the solution pH up to 5;
allowing precipitates to form and settle down at the bottom;
extracting the precipitates; and
dissolving 20-30 gm/L of precipitates in 6-12 ml/L of Phosphoric acid at a
Ph of 2.02-2.08 and temperature of 65-75 (0C)
2. The process as claimed in claim 1, wherein waste pickling liquor (WPL) is generated when defective galvanised material is treated in acid solution to remove defective coating from steel surface at tube and wire galvanising plant.
3. The process as claimed in claim 1, wherein waste pickling liquor contain Zinc in wt.%/L 95-110, Iron 8-15 wt.%/L.
4. The process as claimed in claim 1, wherein the phosphate coating composition comprises Zn in wt.% 20-40, Iron in wt.% 6-18, phosphorus in wt. % 7-16 and O in wt.% 30-45.

5. The process as claimed in claim 1, wherein metallic substrate includes
TMT& CTD bars, wires, tubes, sheets, rectangular & square shape of
structural members.
6. The process as claimed in claim 1, wherein surface roughness of the
coating varies in range of Ra 2.1 to 2.38.