FIELD OF THE INVENTION
The present invention relates to an improved process for batch hot-dip galvanizing of steel tubes in which use of hazardous flux materials is eliminated to achieve environmental safety including reduction in zinc wastage produced as by product.
BACKGROUND OF THE INVENTION
Galvanized steel material is widely used where the material is exposed to a corrosive atmosphere or other corrosive environment. Galvanising process can be of two types; e.g. batch galvanising and continuous galvanising. Batch galvanising process is performed open to atmosphere as opposite to continuous galvanising where an inert atmosphere prevailed. In batch galvanising, the material is first cleaned from oil and grease by alkali cleaning method and then the surface oxides are cleaned using acid pickling. Therefore these clean surfaces are very much susceptible to oxidation. Conventionally, these materials are temporarily protected by dipping them in a solution of zinc ammonium chloride of various solution strengths. These flux coated tubes are dipped in molten zinc or alloy bath for galvanising. The use of flux material is associated with several hazards. Once entered into the bath, the flux materials are dissociated and fumes of toxic gasses like ammonia and hydrogen chloride come out from the bath. Moreover as these flux materials dissociates within the bath, they accumulate inside the bath and molten zinc gets wasted in a large extent (as given in Table I). Hence a new method of batch galvanising is proposed to get rid of environmental hazards as well as minimizing the zinc loss as by-products.

The known processes of galvanising using metallic deposition prior to hot dipping are captured below.
Method of transforming a metallic zinc coating on at least one side of a ferrous metal sheet or strip into a surface coating of a zinc-iron alloy and a product so formed is known from US 4285995.
Since one surface of the steel sheet material used for automobile and truck bodies generally has one side thereof painted or welded and the other side exposed to a highly corrosive environment and since a metallic zinc surface coating is not readily painted or weldable, it has been found desirable to provide one surface of a zinc coated steel strip with a surface which is free of metallic zinc. The process for converting a zinc surface coating into an iron containing alloy coating to improve the paintability and weldability thereof is known from prior art. US Patent Nos.4,171,391; 4,171,394; and 4,120,997, teach processes for producing galvanized ferrous metal sheet material having at least one side formed with an iron containing alloy surface coating.
In order to produce a galvanized steel sheet having a coating of iron containing alloy on the surface, according to the prior art, it was necessary to apply considerable energy in the form of heat on the galvanized surface in order to convert the zinc surface coating into an iron containing alloy surface coating. However, the step of heating a zinc surface to form an iron containing zinc alloy surface coating is de-limited by the parameter namely, the line speed either in a process of producing a zinc-iron alloy coating on one-side only or on both surfaces of a galvanized ferrous metal strip. One known way to reduce the amount of thermal energy and time required to convert a metallic zinc coating on

a ferrous metal strip into an iron containing alloy surface coating, is to ensure that there is no unalloyed metallic zinc remaining in the surface of the coating, and simultaneously avoiding formation of excessively thick brittle subsurface zinc-iron alloy layer on the other surface of a galvanized ferrous metal strip.
It is known in the art that the rate of diffusion of iron from a ferrous metal base into a zinc hot-dip coating (i.e. the zinc-iron alloy growth rate) can be very significantly increased and thereby significantly reduce the amount of heat required to transform a metallic zinc coating of a given thickness into a surface coating which is entirely free of metallic zinc by subjecting a ferrous metal strip after surface cleaning and before hot-dip coating to a pregalvanizing surface treatment which comprises applying an ultra thin flash film or coating of metallic copper to the clean ferrous metal surface of the strip and thereafter heating the copper coated strip to a temperature of between about 704°C. (1300° F.) and 927° C. (1700° F,) in non-oxidizing atmosphere, such as a reducing atmosphere conventionally used in a Sendzimir-type process for preparing a strip for hot-dip coating. The strip is then preferably cooled to about the temperature of the hot-dip coating bath before immersing the strip in the hot-dip coating bath. When a ferrous metal strip pretreated in the above manner is immersed in a hot-dip zinc coating bath an intermetallic zinc-iron alloy layer is formed at a substantially greater rate than when the strip has not received a flash coating of metallic copper and heated to a temperature of between about 704° C—927° C. (1300°F.—1700° F.). Thus, less heat is required to effect the complete alloying of the metallic zinc in the hot-dip zinc coating. And, when alloying a very thin metallic zinc coating, it is unnecessary to apply additional heat to the hot-dip coating bath in order to avoid having unalloyed metallic zinc in the surface of the coating.

The non-patent literature published in Metallurgia Italiana 2002, 94(11-12), pages 19-24 describes process for thin film copper plating as steel pretreatment for bath hot dip galvanizing in Zn-AI bath. In addition to controlling the galvanized coating thickness, it is known that the aluminum content of the coating bath can be controlled in order to efficiently convert a thin zinc coating into the desired alloy surface coating without forming an excessively thick zinc-iron intermetallic alloy coating on the opposite heavier zinc coated side of the strip. A conventional galvanizing coating bath will contain between about 0.18— 0.20 wt. percent aluminum in order to inhibit the formation of a thick zinc-iron intermediate alloy layer between the surface of the strip and the surface of metallic zinc. When it is desired to convert at least one of the zinc coatings into an alloy surface coating, the aluminum content of the coating bath is reduced to between about 0.10 to 0.16 wt. percent aluminum. When the hot-dip coating bath contains an aluminum content of 0.14 wt. percent aluminum, thin zinc coating treated on a steel strip can be rapidly converted to an alloy coating without forming a thick sub surface zinc-iron alloy coating on the opposite side of the hot-dip coated strip and without requiring any major alterations in the operating conditions of a conventional continuous galvanizing line.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved process for hot-dip galvanizing of steel tubes which eliminates the disadvantages of prior art copper coating and rinsing and drying and finally hot dip galvanizing.

A still another object of the invention is to propose an improved process of hot-dip galvanizing of steel tubes, which replaces the fluxing step in batch galvanising by a step of copper flash coating implemented at room temperature without application of any external current source.
Yet another object of the invention is to propose an improved process of hot-dip galvanizing of steel tubes, in which the copper-coated steel tubes are galvanized to decrease the environmental hazards caused by conventional fluxing materials.
A further object of the invention is to propose an improved process of hot-dip galvanizing of steel tubes to eliminate the fluxing step in batch galvanising in which the copper coated steel tubes contributes in reduction of zinc wastage as by-product as compared to the conventional flux materials.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved process for hot-dip galvanizing of steel tubes comprising the steps of:
alkali cleaning of tubes for 5-30 minutes at a temperature of 50-70°C
at ambient pressure;
pickling the cleaned tubes using 8-12% hydrochloric acid for 2-30
minutes at a temperature of 35-65°C at ambient pressure;
rinsing the pickled tubes in water at ambient temperature;
applying coating of copper on the tubes by dipping in a solution having
5-10% copper sulfate and 2-15% sulfuric acid for 5-50 seconds at
ambient temperature and pressure;

rinsing the copper coated tubes in a water tank; drying the copper coated tubes in a furnace electrically heated for a residence time of 2-15 minutes at a temperature of 70-80°C; and dipping the copper coated tubes are dipped in molten zinc or alloy bath kept at a temperature of 460-470°C for 50-100 seconds.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a flow sheet depicting the steps of inventive process.
Figures 1A - lA(a) to Figure lA(c) - pictorially represent a comparison of coated samples of prior art and inventive processes in respect of:
(a) salt spray corrosion behavior,
(b) write rust comparison, and
(c) red rust comparison,
Figure 2 - shows a comparison between prior art and inventive sample in respect of cross section of the coating after galvanizing.
Figures 3 (a) & 3(b) - respectively shows inventive and prior art samples in respect of coating adhesion.
Table 1 - Shows an estimation of economic significance of the invention compared to prior art.

DETAIL DESCRIPTION OF THE INVENTION
According to the invention, the following steps for metal flash coating of steel tubes are implemented including conducting a characterization analysis between the prior art coated tubes and the tubes coated under inventive process:
Pretreatment of samples
Prior to copper flash coating process, the tube samples are cleaned following the conventional practice for example, alkali cleaning and acid pickling. As the scales on the steel surface are removed after pretreatment, tube surface becomes very prone to oxidation. So immediately these pickled tubes were flash coated by dipping in copper solution.
Copper flash coating
Copper flash coating is performed at room temperature. This being an electroless process, no external current supply is required for copper coating. A plating bath is prepared having a composition of 5-10% copper sulfate (CuS04). The time of flash coating is varied from 5 to 50 seconds with an interval of 5 seconds. After completion of the copper flash coating, the samples are washed / rinsed in water and dried.

Hot dip galvanizing
The copper coated tubes were galvanized afterwards. The zinc melting furnace is electrically heated. Inside the furnace, a stainless steel pot is interposed, which accommodates at least one graphite crucibles. Solid zinc blocks are melted and the bath temperature maintained at 460-470°C. Sufficient time is allowed to melt the zinc completely. The tube samples are dipped in liquid zinc and caused to move upwardly and downwardly by a pulley-motor system. The samples are kept for 90 seconds inside the bath and then pulled outside.
Characterization and property evaluation of the sample processed according to the inventive process.
Properly cut and prepared coated samples were analyzed by different techniques to examine the coating thickness, composition etc. Moreover, performance tests were also carried out between a laboratory-scale coated sample and plant coated sample to evaluate the corrosion resistance, coating adhesion and associated properties.
Surface and Cross sectional analysis
For analysis of the surface morphology, a plurality of samples were cut into small pieces of 10mm x 10mm sizes. The top surfaces of the coated tubes were examined using SEM and the elemental composition was also determined using EDS analysis. For a comparative study, the plant samples were also studied.

No significant difference in coating surface morphology and composition was observed between the plant and lab samples.
Samples of 10mm x 2mm size were used for cross sectional analysis conducted by EDS technique, both by point and area analysis. The coating thickness was found to be about 40 micron for both plant and experimental tubes. A similar amount of Fe-Zn alloying was also observed in both types of samples through EDS and elemental mapping analysis. A small amount of copper was found to be dispersed throughout the coating indicating a diffusion of copper all along the coating (Figure 2).
Corrosion test SST
All the samples were exposed to salt spray chamber at 5% NaCI using the ASTM B117 method. The salt spray tests were performed to check the comparative corrosion performance of plan coated tubes and also experimentally coated tubes
(Fig. la). Samples of almost same dimensions were exposed to corroding chamber. 100% white rust was observed at 24th hour for plant samples and at 48th hours for lab coated samples (Fig. 1(b)). 10% red rust was obtained for the plant samples after 65 days (1560 hours), whereas for lab coated samples, red rust was not visible (Fig. 1(c)).
Bend test
To determine the adhesion of coating, experimental and plant tube samples of a fixed length were cut into samples of 2mm width. These strip like samples were

then pressed at their centre until the two sides formed an angle of 90°. These bent samples were then again flattened applying load. Then, the samples were again bent applying force in the opposite direction and re-flattened. After the experiment was over, the area of loading was examined to check whether there was any crank or surface pealing. The comparative results are displayed in Fig. 3.
It was observed that after passing through the severe conditions of bending, there was no peeling off from the coated substrate both from the plant sample and lab coated samples. Thus, it can be concluded that the coating adhesion of lab coated samples was at par with the plant samples.
Bath sample analysis
Zinc bath samples before and after the dipping of copper coated samples were analyzed. Molten zinc, both fresh and after experiments, was taken out from graphite pot and small chips were grilled from the bath samples. These solid chips were analyzed by ICP method to find the elemental composition of the bath samples.
It was seen that there was hardly any deposition or contamination of copper inside the bath. It can therefore be concluded that the copper was not detrimental to the bath chemistry and can be used to control the huge amount of zinc loss due to by-product generation inside the molten zinc bath, as perforce to be accepted in prior art.

WE CLAIM:
1. An improved process for hot-dip galvanizing o steel tubes comprising the steps of:
alkali cleaning and acid pickling of the steel tubes;
preparing a plating bath having a composition of 5 - 10% copper sulphate and 2 to 15% of sulfuric acid;
conducting copper flux coating of the cleaned tubes at room temperature for a period 5 to 50 seconds;
cleaning, washing, and drying of the copper flash coated tubes; and
- hot-dip galvanizing of the copper coated tubes in an electrically heated zinc melting furnace, the temperature of the bath being maintained at about 460-470° C and the tubes are immersed in the liquid inside the bath for a period of about 50-90 - seconds.
2. The process as claimed in claim 1, wherein the alkali cleaning of the tubes is carried out for a period of 5-30 minutes at about a temperature between 50-70°C at ambient temperature.

3. The process as claimed in claim 1 or 2, wherein the pickling of the cleaned tubes is carried out for about 2 to 30 minutes at a temperature between 35 to 65°C at ambient pressure in a hydrochloric acid solution having 8 to 12% hydrochloric acid.
4. The process as claimed in claim 1, wherein the copper coating of the cleaned and pickled steel tubes is carried out by dipping the tubes in a solution having 5 to 10% of copper sulphate and 2 to 15% sulphuric acid.
5. An improved process for hot-dip galvanizing of steel tubes as herein described and illustrated with reference to the accompanying drawings.

ABSTRACT

The invention relates to an improved process for hot-dip galvanizing of steel tubes comprising the steps of alkali cleaning and acid pickling of the steel tubes; preparing a plating bath having a composition of sulfuric acid; conducting copper coating of the cleaned tubes, washing and drying of the copper flash coated tubes; and hot-dip galvanizing of the copper coated tubes. The process of galvanising ensures less environmental hazards and less wastage of zinc as by-product.