Inspection & Testing Guide Line For Galvalume Coil/Sheet

  1. Quality Assurance Plan
  2. Inspection
  3. Chemical Testing
  4. Mechanical Testing
  5. Metallurgical Testing
  6. Corrosion Testing

Chemical Testing

This method is developed for Al/Zn bath which is nominally 45% Zn and 55% Al. The method may not be suitable for alloys having significantly different Al-Zn composition.

The sample of pot metal, coating or dross is dissolved, diluted to volume and then aliquots are taken for each analysis.

Silicon is determined gravimetrically by determining the mass of insoluble material in the Al/Zn alloy filtration.

Zinc is determined by titrating at pH 5-6 with standard EDTA (Ethylenediaminetetraacetic acid solution) solution.

Aluminum cannot be directly titrated with EDTA using xylenol orange indicator, since the metal indicator complex is more stable than the EDTA complex. Therefore to determine aluminum’s Excess EDTA is added to react with the total metal ions present. The uncomplexed EDTA is back titrated   with standard zinc solution. Iron is determined by titration with standard potassium dichromate solution. Iron in dross is determined by an alternate AA method.

Sampling

Pot Samples. Drill from the bottom of the sample only, to avoid any dross which may be on the top surface. Take drillings from a couple of places from around the pot sample. Drill a small amount first and discard these drillings. Keep the subsequent drillings for analysis.

Coating Samples. Samples should be 5.05 cm diameter discs for preference. The surface should be clean and free of crayon markings etc. 

Dross – use a clean 7 mm drill to obtain a whole core sample in three separate locations.

Reagents and Chemicals

  1. Hydrochloric acid (conc.)
  2. Hydrochloric Acid (1+1)
  3. Inhibited hydrochloric Acid solution
    1. To a 2000 ml of (1+1) Hydrochloric acid solution add 2 ml of Rodine 1151.
  4. Inhibited hydrochloric Acid Solution
  5. Xylenol Orange Indicator
  6. Hydroge Peroxide (1+1)
  7. Hexamine (Hexamethelenetetramene)
  8. Chloroform
  9. Acetyl-acetone
  10. Acetyl-acetone Reagent
  11. Standard EDTA salt solution – 0.1 N
  12. EDTA Disodium salt Reagent 0.06N
  13. Mercuric chloride
  14. High purity Al foil, high purity iron, high purity zinc drillings
  15. Synthetic Al-Zn alloy standard solution.

Take 2.65 g of high purity zinc, add 3.65 g of high purity Aluminium in a 1 litere beaker. Pitette 10 ml of standard iron solution into the beaker and add 200 m of hydrochloric acid. Add 2 drops of saturated mercuric chloride solution to catalyze the dissolution of aluminium. Cover the beaker and warm gently until dissolution of aluminium commences. Remove from hot plate and allow the metal to completely dissolve. Cool and dilute to one liter in volumetric flask.

  1. Standard zinc solution – 12.12 mg/lit
  2. Standard Iron Solution – 10 mg/lit
  3. Ammonium Fluoride  NH4F – 160 g/l
  4. Phosphoric Acid
  5. Stannous Chloride Solution – SnCl2.2H2O
  6. Standard Potassium Dichromate Solution – 0.01N
  7. Sodium Diphynyl Amine Sulphonate (tablets)
  8. Burette, B grade , 25 ml with 0.05 ml graduations.

PROCEDURE

Determination of Silicon

  • If analysing bath metal, weigh out accurately 1.250 g sample into a 400 ml beaker.  For dross, use whole of core sample and weigh to three decimal places.Add 30 ml of 1:1 hydrochloric acid to dissolve the sample (60 ml for dross).  Cover the beaker with a watchglass to prevent mechanical loss.If analysing coated samples, use enough discs to give about 1.25 g of total ZINCALUME coating (3 discs of 5.05 cm diameter are usually sufficient). Weigh each disc, then strip the coating by immersing each disc consecutively into a beaker containing 60 ml of 1:1 inhibited hydrochloric acid.  Remove each disc, and wash the surfaces with a minimum amount of distilled water from a squeeze bottle with washings returned to the beaker.  Reweigh the discs after drying to determine total amount os Al-Zn alloy coating.
  • After dissolution of coating or bath metal, add 5-6 drops of 1:1 hydrogen peroxide solution (10 drops for dross) and heat the covered beaker to boiling to ensure complete oxidation of iron.
  • Simmer for 2-3 minutes, then allow to cool.
  • Filter through a 5.5 cm Whatman 541. Wash through alternately with water and 1+1 hydrochloric acid twice and finally with water
  • Place the filter paper with residue in a preweighed platinum crucible and place in the muffle furnace at 600°C for 30 minutes. After cooling reweigh the crucible and calculate the silicon level as outlined in section 5.0.
  • For pot samples and coating, dilute the filtrate from step 4.1.6 to 200 ml and reserve 2 x 50 ml aliquots for determination of Zn and Fe and a 25ml aliquot for Al.
  • For dross, dilute the filtrate from step 4.1.6 to 1 litre and reserve 2 x 100 ml aliquots for zinc and 2 x 50 ml aliquots for aluminium. Retain solution for iron determination by AA.(method WQID10-C125P).

Determination of Zinc

  • Transfer a 50 ml aliquot from section (h) (100 ml dross) to a 500 ml stoppered conical flask and dilute to 150 ml. With every sample or batch of samples carry out an assay of the synthetic
  • Al-Zn standard solution as a check on technique. Add 25 ml of acetylacetone reagent, stopper the flask and shake for 30 seconds to extract iron and aluminium. (Note 8.2) The water phase should be pink in colour and the solvent layer yellow.
  • Add sufficient solid hexamine to give a pH of 5-6 , then shake the flask contents to ensure all the hexamine is dissolved.
  • Add 2ml of Xylenol orange indicator and shake flask contents.
  • Titrate with standard 0.1N EDTA solution until the test solution, viewed from the side, against a white background, changes from red to yellow. 
  • Calculate zinc level as outlined in section 5.0.

Determination of Aluminium

  • Transfer a 25 ml aliquot from section (h) to a 500 ml conical beaker, and add100 ml of EDTA disodium salt reagent, using a measuring cylinder. With every sample or batch of samples carry out an assay of the synthetic Zincalume standard solution as a check on technique..
  • Add solid hexamine to bring the pH to 5-6 (Note 8.7) add a few boiling chips and boil on a hot plate for 5-10 minutes (Note 8.8)
  • Cool.  Add 2 ml xylenol orange indicator and titrate with standard zinc solution to a red endpoint.  (Note 8.9).
  • Add 25 ml ammonium fluoride solution, return beaker to the hot plate and boil for 5-10 minutes. (Note 8.10)
  • Cool. Titrate with standard zinc solution to a deep pink endpoint and record the titre.
  • Determine the aluminium level as outlined in section 5.0.

Determination of Iron.

  • Transfer a 50 ml aliquot from section 4.1.8 to a 500 ml conical flask. With every sample or batch of samples carry out an assay of the synthetic ZINCALUME standard solution as a check on technique.
  • Add 15 ml conc. hydrochloric acid and warm on a hot plate.  Add 100 ml water and bring to the boil.
  • While boiling gently, add stannous chloride solution dropwise until all the iron has been reduced (yellow ferric colour disappears).  Add 2 drops in excess.
  • Cool.  Add 5 ml conc. phosphoric acid and 5 ml of saturated mercuric chloride solution mixing well.
  • Add a sodium diphenylamine sulphonate tablet and titrate with standard potassium dichromate solution to a purple endpoint.
  • Determine the iron level as outlined in section 5.0

Determination of Chromic( Cr+6 &Cr+3) on strip :tions

Procedure: Take out 80mm circle sample from GL sheet. Wash the sample in 50ml hot water, then dilute this solution up to 100ml for Cr+6 on both sides.

Dissolve the same sample in 1:1 HCL till zinc stripping. Then dilute this solution up to 100ml for Cr+3 on both sides. Then test both samples with AAS(Follow Work Instruction no : (J-QC-WI-04).

Cr+6 on Strip (mg/m2): AAS reading in ppm X 20

            Cr+3 on Strip (mg/m2): AAS reading in ppm X 20

Total Cr on Strip of both sides (mg/m2)= Cr+6 + Cr+3

          B) Multi Element Benchtop X-Ray Spectrograph(Portaspec)

          Portaspec WDXRF is used for measuring coating weight of chrome based pre treatment. The X-ray tube directs a beam of primary radiation onto a sample of the material under analysis. The primary radiation causes the sample to emit a secondary radiation that consists a characteristic line of each element present in the sample. The characteristic line of each element represents a wavelength, which varies in a regular fashion from one element to another. The characteristic wavelength decreases as the atomic wavelength of the element increases.

  1. For every operation, the instrument is calibrated first with standard Chrome based sample.
  2. A blank sample without any surface treatment on GL sample is inserted to obtain “0” counts. The sample size is 50 mm diameter circle or equivalent area.
  3. The GL sample under test is inserted to obtain chrome content on either side. Portaspec measures the counts and gives result in terms of mg per meter square of Chromium.
  4. The acceptance norms for AFP is 12 – 20 mgsm each side.

C) Weight Difference : For total coating Weight – Method – 1

  • Take initial weight of sample of 8m mm diameter as W1.
    • Clean the sample from either side with acetone till the plastic like layer is completely removed.
    • Take final weight as W2.
    • Coating weight on that side = (W1-W2)*199.04  gsm.

   D) Weight Difference :For total coating Weight – Method – 2

  • Note weight of 80 mm dia circle as W1
    • Heat the sample in muffle furnace upto 450 ºC for 10 minutes. The AFP layer burns completely.
    • Rewigh the sample and note the final weihght as W2.

Coating weight on that side = (W1-W2)*199.04  gsm

Mechanical Testing

Hardness Test

Hardness is resistance offered by a material for indentation. American Society for Testing of Materials has defined it as “A number related to the impression made by an indenter of specific size and shape under a known load.”  

For thk <= 0.25 mm – Test is one on Vicker’s  Hardness Testing Machine

For thk 0.26 – 0.60mm         – 30 Kg load is applied on the  Rockwell Hardness Testing  machine

For Thk >0.60 mm                – 100 Kg load, Rockwell       Hardness testing machine

Tension Test

Ref. Standard – ASTM E8 – 01

Tensile Strength (UTS) is the stress corresponding to maximum load on the Stress –Strain Curve.

Mathematically the Tensile Strength, (UTS) is calculate as

                                       P(max)

            UTS =             —————-

                                          A0

P = Load

A0 = Initial Cross Section Area

Yield Strength (YS) is the stress required to produce small-specified amount of plastic deformation.

                                            P(yield)

YS =                           —————-

                                          A0

P = Load at yielding

A0 = Initial Cross Section Area

The usual definition of this property is the offset yield strength determined by the stress corresponding to the intersection of stress strain curve on a line parallel to the elastic part of the curve offset by 0.2% stress.

Elongation (EL) is the Change in the gauge lengthat fracture or the reduction in area at fracture. It is the measure of ductility of a material.

                                    (Lf – L0)

            %El =           —————- X 100

                                          L0

L f = Gauge length after fracture

L0 = Initial Gauge length

Sampling (Ref. Standard – ASTM A 924)

Specimen should be 50 to 100 mm wide. Specimen should be cut not less than 50 mm from the edges.

Coating Weight (Mass) Test

For measuring the coating mass, Weigh-Strip-Weigh method is used. It is a destructive test that determines the coating weight by measuring the difference between coated and uncoated sample. For width > 450 mm with, three specimens are taken, one from each edge.  The edge sample should be taken from >50 mm from edge.

Testing Procedure:

a) Sample weight with coating (W1) is taken on the electronic balance in gms. after  cleaning the  sample.

b) Coating shall be removed by dissolving in above stripping solution and rinsed with water/alcohol clean with dry clothes. Weight (W2) sample is taken again.

Coating wt calculation (gms/ M2):

For non std. Size sample = Wt diff.( w1-w2)gms*1000/area,  

Where area  = w 2 gms.)/(Base metal thickness (mm) *7.85)

Lock forming Test

Scope : To prevent poor adherence material from being unknowingly produced & sent to customers.

Definition : The forming of two adjacent edges prior to interlocking.

Sample

The test piece should be in transverse direction i.e.across the strip and should be minimum 200mm in width. Refer Fig 3

Test Principle -The test piece is passed through a Lockformer machine.This stimulates forming under severe conditions & allows assessment across the entire width of the strip

lock forming machine

Fluting Test

The tendency to form parallel kinks or creases in sheet metal during curving or bending operations.  The creases occur at right angles to the direction of curving and are associated with the non-uniform yielding of metal causing localized deformation.

2.4.5.1 Sampling 

One per CR coil.

Test Method

One GL sample cut from above sample is made coil form on 3 inch dia./or as per mandrel diameter specified by th customer.  Appearance of wrinkles are verified according to parameter.

Salt Spray Testing

Scope : The purpose of corrosion test is field corrosion performance of the product.

Procedure

Basically salt spray test procedure involves the spraying of a salt solution onto the samples being tested. This is done inside a temperature-controlled chamber. The solution is a 5% Salt solution (Sodium chloride-NaCl) solution. The samples under test are inserted into the chamber, following which the salt-containing solution is sprayed as a very fine fog mist over the samples. The temperature within the chamber is maintained at a constant level. Since the spray is continual, the samples are constantly wet, and therefore, constantly subjected to corrosion.

The most common procedure is the test described in ASTM B 117.

Standard practice for operating salt spray (fog) Apparatus.

  1. Racks are contained in salt spray chamber (3’high, 3’deep, 5’wide)
  2. Place the samples on the rack at a small tilt angle.
  3. 5%NaCl in tap water pumped from reservoir to spray nozzles.
  4. Solution mixed with humidified compressed air at nozzles.
  5. Compressed air atomizes NaCl solution into a fog at nozzles.
  6. Set the bath temperature between 410C to 450C.

Put off electrical switch before opening the chamber to avoid shock.

Within the chamber, the samples are rotated frequently so that all samples are exposed as uniformly as possible to the salt spray mist.

The corrosion performance is rated in following ways.

  1. Number of hours until rusting of the steel is first evident.
  2. Number of hours until 5% of the surface area is rusted.
  3. Number of hours until 10% of the surface area is rusted.

The onset of red rust on sample means that the coating has been consumed by the corrosion reaction, the corrosion of base steel is beginning. There is no one best performance criteria. It simply depends on what user defines as failure.

Salt spray test resistance of zinc containing hot dip coatings is as follows.

For Galvanise Z 275gsm coating approximate time to 5% red rust is 205hrs.

For Galvalume AZ50 coating approximate time to 5% red rust is 1075hrs.

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