Hot dip galvanizing procedure.
All items to be blast cleaned as per ISO 8501-1.The surface profile shall be restricted to a maximum of 30micron.Prior to blasting,all rough welds,weld spatter and other sharp projections etc.shall be ground smooth.All oil,grease or any other impurities on the blasting surface shall be removed by using an approved solvent prior to blasting.
The equipment & tools used for cleaning and surface preparation shall be of suitable design and quality to complete the works strictly as per specification.Where compressed air is used,air lines shall be provided with effective,well maintained oil and water traps.Efficient well maintained air filters shall be provided to control the dust.
The blasting medium shall be capable of giving the required surface profile under nozzle pressure of 100psi.The blasting medium shall satisfy the quality levels required.Steel grit/garnet shall be used for surface preparation.All blasting pots and compressors shall be fitted with efficient moisture traps.Only tungsten carbide venture type nozzle shall be used.
The abrasive shall be dry,salt free,free of impurities and inclusion and with a neutral pH value.Test certificates shall be provided.The abrasive shall be covered properly to be free from contamination prior to use.No blasting operations shall be carried out to the detriment of the progress or freshly applied coating which has not dried thoroughly.
Hot dip galvanizing materials preparation for application.
Material Inspection-All hot dip galvanizing material received shall be inspected and weighed prior to taking up for hot dip galvanizing.
Degreasing-Hot dip galvanizing materials shall be immersed in a solution of sodium metasilicate anhydrous diluted with water in the ratio of 1:10 for removal of oil and grease for approximately 2 hours.The remaining loose millscale shall be removed by wire brushing.
Water Rinsing-The hot dip galvanizing material shall be immersed in water bath to ensure all chemical are removed.
Pickling-The hot dip galvanizing material shall be immersed in a bath containing hydrochloric acid diluted with water in the ratio of 1:0.5 for a period of 30 minutes to obtain a clean and smooth surface.All millscale,rust etc.shall be removed by this operation.
Water Rinsing-The materials shall be immersed in water bath for approximately 1 minute to ensure all acid residual is removed.Then the material shall be rinsed clean with fresh water using suitable hose.
Hot dip galvanized application.
Fluxing-Immediately after removing from the water bath the material shall be immersed in the bath containing a solution of ammonium chloride for a period of approximately 5 minutes.Brackish water shall not be used for fluxing.
Drying-Material shall then be dried by using compressed air.
Galvanizing-After drying operation,the material shall be dipped in the molten zinc bath for the required time to obtain the correct thickness of coating.Prior to galvanizing operation,a test piece of the same material and thickness which had undergone all the procedures shall be dipped in galvanizing bath to the predetermined time and thickness checked to fix the correct galvanizing time for that particular batch of material to achive the required thickness of coating.
Aluminium added to the bath shall never be more than 0.15%.The water used in the whole process shall be de-ionized water.The time to be kept in the hot bath shall be calculated prior to loading the materials to the bath and recorded in the report format.Zinc used in galvanizing bath shall confirm to specification.The moltan metal in the working volume of galvanizing bath shall contain not less than an average volume of 98% zinc by weight.
The trace elements (aluminium,nickel and tin) can be added to the galvanizing bath only as part of a pre alloyed zinc feed,or can be added to the bath using a master feed alloy to enhance the cosmetic appearance of the finished product.However,the total of impurities in the molten zinc shall not exceed 1.5% by mass.The galvanizing bath temperature shall be maintained at 450°C TO 460°C.
Quenching-After galvanizing the material shall be immediately quenched in the water bath to improve the zinc coating mechanical properties.
Thickness measurement of galvanized materials.
The thickness of the coating shall be determined by magnetic thickness gauge measurements in accordance with practice ASTM E 376.Thickness gauge elcometer model:254F shall be used for this purpose.A minimum of five readings shall be taken (in each location) near the ends and in the middle of each piece being tested and the average of the thickness values taken at each location shall not be less that the specified values.
The average coating thickness for the itms shall be the average of value obtained in the three locations shall be not less that specified value.A minimum of five magnetic test readings shall be taken on 1000mm² of coated areas.The mean thickness value over the whole of such five readings shall be minimum100µ.However,this mean thickness of any items shall never be more than 250µ.The thickness measurements shall not be taken on cut surfaces or areas less than 10mm the edges.A control sample for thickness testing shall be taken randomly from each lot of all the hot dip galvanizing items.