ISO-10546-1993.pdf

INTERNATIONAL STANDARD IS0 10546 First edition 1993-04-01 Chemical conversion coatings - Rinsed and non-rinsed chromate conversion coatings on aluminium and aluminium alloys Couches de conversion chimique - Couches de conversion au chromate fin a predrying not exceeding 65 “C and a second drying at 100 “C to 110 “C. in order to achieve the best adhesion. NOTE 1 Heat treatment at 100 “C to 110 “C interferes with the corrosion resistance on non-lacquered areas. the electrical resistance between an electrical contact and the aluminium to a very small extent. When measured at 9 V open circuit voltage and 2 A current according to IEC 130-1, the resistance should be less than 0,l 52. Highly coloured brown, yellow or green coatings show a marked increase in electrical contact resist- ance, with increasing mass per unit area of the chromate layer. 6.3 Adhesion The coatings shall be adherent and non-powdery. There are no practical tests for measuring the ad- hesion of a chromate conversion coating on alu- minium. However, a practical evaluation of the adhesion can be made by measuring the adhesion of a secondary organic film applied to the chromated aluminium. When specified, the chromate conversion coating shall pass the organic coating adhesion test specified in IS0 2409. 6.4 Corrosion resistance When subjected to the neutral salt spray test speci- fied in IS0 9227, three separate test specimens of the coating shall withstand exposure for the duration shown in table 1, without giving evidence of more than a total of eight isolated spots or pits to the un- aided eye. None shall be larger than 1 mm in diam- eter. Each individual test specimen shall have not more than five isolated spots or pits, none larger than 1 mm in diameter, on their respective surfaces. Spots within 10 mm of the edges of the test specimens are not counted. As an alternative method of testing cor- rosion resistance, test D of IEC 68-2-3 is applicable. 6 Coating requirements 6.1 General 6.5 Coating mass per unit area The mass of the coating per unit area of coated sur- face shall conform to the values given in table2. any tests (including corrosion tests) shall be deferred until the expiry of that period. Chromate conversion coatings harden with age bv gradual dehydration. They should, therefore, be han- dled carefullv for the first 24 h after treatment, and It should be recognized that heavier coating masses do not always provide better performance, particularly The procedure specified in IS0 3892 shall be used. 6.5.1 Coating mass significance The green chromate-phosphate coatings achieve their maximum corrosion resistance after a longer storage period, usually 1 month to 2 months at room tem- perature. when using the coating for an organic film or adhesive base. 6.6 Coating identification 6.2 Electrical insulation resistance Colourless, light yellow or light green iridescent chromate layers, of low mass per unit area, increase The presence of chromium in conversion coatings and the identification of the appropriate class can be con- firmed by their visual appearance and by the tests described in annex A. -,-,- IS0 10546:1993(E) Table 1 - Relative corrosion resistance Coating class) Exposure timez), h Cast alloys with a nominal Heat-treatable alloys and Non-heat-treatable cast alloys with a nominal silicon content greater wrought alloys silicon content less than than 1 % or equal to 1 % 1 500 336 48 2 250 168 24 3 168 120 12 4 500 336 48 I) Class numbers are assigned on the basis of traditional usage and do not reflect a ranking of corrosion resistance. The specified exposure time is only effective for a chromated surface without additional coatings. The test gives no information on the performance of a surface which is additionally coated with organic film, 2) The exposure times are indicative of the relative corrosion resistance of the various coating classes on the different alloys, but no direct relationship exists between the performance in the neutral salt spray test and performance in service. Table 2 - Classification of chromate coatings Class Appearance Coating mass per unit area g/m2 Corrosion protection 1 Yellow to brown 1.3 to 3 Maximum corrosion resistance, gener- ally used as final finish 2 Yellow (light to I 0,2 to 1,3 Moderate corrosion resistance, used iridescent) as a paint base and for bonding to rub- ber I I I 3 Colourless 0,05 to 0.2 Decorative, slight corrosion resistance, low insulation resistance 4 Green 5 Light green 6 Colourless 2 to 5 0,2 to 2 0,05 to 0,2 Moderate corrosion resistance, used as a paint base and for bonding to rub- ber NOTE - No relationship exists between class number and degree of corrosion resistance. Class numbers have been as- signed on the basis of traditional usage and for compatibility with national standards. The coating mass should be measured as described in IS0 3892. 6.6.1 Chromate coatings (classes 1 to 3) The determination of the presence of chromium and the absence of phosphate in the coating will identify the layer as a chromate conversion coating. The test described in A.2 and A.3 shall be used. 6.6.2 Chromate-phosphate coatings (classes 4 to 6) It is necessary to determine the presence of phos- phate and chromium and the absence of zinc, in order to distinguish chromate-phosphate coatings from anodized coatings or zinc-phosphate coatings. The test described in A.2 and A.3 shall be used. 7 Sampling and test specimens Unless otherwise specified, the sampling plans of IS0 4519 shall be used to test the coatings. The test specimens should be of the same alloy and surface condition as the article they represent. They should measure 100 mm x 150 mm. 3 -,-,- IS0 10546:1993(E) 8 Classification The chromate finishes applied can range from brown, thick coatings providing maximum corrosion pro- tection, to yellow, intermediate thickness coatings suitable for organic film bases, or to colourless, thin coatings suitable for lowest insulation resistance. The yellow coatings vary from golden yellow to iridescent light yellow. The chromate-phosphate finishes applied can range in colour from green to iridescent light green. Finishers can seldom guarantee to supply exact shades of colour with chromate conversion coatings. If it is necessary to have exact shades of colour, it is possible to dye chromate coatings, having a coating mass greater than 0,4 g/m*, to obtain a wide range of colours, but they can only be expected to give an order of added corrosion resistance similar to that provided by the undyed coatings. It should be noted that colour and colour uniformity will vary somewhat between one alloy and another, and from a polished surface to an etched surface. lridescense and vari- ations in colour density from one area of the surface to another are normal, and should not be considered a sign of poor quality. The finishes are divided into six classes; their most important characteristics are listed in table 2. 4 -,-,- IS0 10546:1993(E) Annex A (informative) A.1 Reagents Qualitative testing of the layer composition During the test, use only reagents of analytical grade and distilled water or de-ionized water. A.l.l Sodium hydroxide (NaOH), approximately 5 % (m/m) solution. A.1.2 Sodium hydroxide (NaOH), approximately 20 % (m/m) solution. A.1.3 Hydrogen peroxide (H,O,), approximately 30 % (m/m) solution. A.1.4 Acetic acid (CHJOOH), approximately 10 % (m/m) solution. A.1.5 Lead nitrate Pb(NO,), approximately 10 % (m/m) solution. A.1.6 Concentrated nitric acid (HNO,), approxi- mately 65 % (m/m) solution (p20 x I,40 g/cm3). A.1.7 Nitric acid (HNO,), approximately 38 % (m/m) solution, made up, for example, by mixing 1 part by volume of concentrated nitric acid (A.l.6) with 1 part by volume of water. A.1.8 Ammonium molybdate reagent. Mix 88,5 g of ammonium molybdate (NH&Mo,02,4H20, 34 ml of aqueous ammonium hydroxide solution with 25 % (m/m) of NH, and 240 g of ammonium nitrate (A.l.9). Dissolve in water by shaking and dilute to 1 litre. A.1.9 Ammonium nitrate (NH4N03). A.l.10 Hydrochloric acid (HCI), approximately 25 % (m/m) solution. A.1 .ll Potassium hexacyanoferrate(ll) K,Fe(CN), approximately 5 % (m/m) solution. A.1.12 Sulfuric acid (H,SO,), approximately 25 % (m/m) solution. A.1.13 Phenolphthalein (C20H,404), approximately 0,5 % (m/m) solution in ethanol. A.2 Testing for chromium A sample of about 300 cm* surface is treated with 50 ml of sodium hydroxide solution (A.1 .I 1, to which 5 ml of hydrogen peroxide solution (A.l.3) was added. The solution, at 50 “C to 60 “C, is poured over the sample, repeatedly if necessary, until the layer is completely removed. The resulting solution is poured off, boiled until all the hydrogen peroxide is destroyed (about 5 min to 6 min), cooled and precipitated with the lead nitrate solution (A.l.5). A yellow precipitate indicates the presence of hexavalent chromium. The smallest quantity of total chromium detectable by this method corresponds to about 5 mg of chromium per square metre of surface. A.3 Testing for phosphate To test for the presence of phosphate in the chromated layer, a sample is taken with a surface to be tested of about 100 cm*. This is treated with 100 ml of sodium hydroxide solution (A.1 .l) at 80 “C to 90 “C until the layer is completely dissolved, or at least until the surface has been clearly attacked. The resulting solution is filtered, and 25 ml of the filtrate are acidified with nitric acid solution (A.l.71, then 10 ml of ammonium molybdate reagent (A.1.8) and 5 g of ammonium nitrate (A.l.9) are added. The sam- ple is allowed to stand for at least 15 min. A yellow precipitate indicates the presence of phos- phate. The smallest quantity of phosphate detectable by this process corresponds to about 40 mg of P,O, per square metre of surface. A.4 Testing for zinc To test for the presence of zinc in the layer, a sample is taken with a surface to be tested of about 100 cm*. This is treated with 50 ml of nitric acid (A.l.6) at room temperature until the layer is either completely dissolved, or at least until the surface has clearly been attacked. The resulting solution is filtered through glass wool, and 25 ml of the filtrate are neu- tralized with sodium hydroxide solution (A.l.2) until 5 -,-,- IS0 10546:1993(E) the phenolphthalein indicator (A.1 .I31 gives a red colouration. The solution is then weakly acidified by addition of about 10 drops of sulfuric acid solution (A.1.12). After addition of 5 ml of the potassium hexacyanoferrate(ll1 solution (A.1 .I I), a greenish- white precipitate indicates the presence of zinc. The smallest quantity of zinc detectable by this process corresponds to about 20 mg of zinc per square metre of surface. -,-,- IS0 10546:1993(E) Annex B (informative) Treatment sequence options The chromate process necessitates a specified sequence of treatment which depends upon the surface condition of the parts to be treated, the aluminium alloy used and the desired appearance of the parts. (See figure B.l .I Figure B.l - Treatment stages of the chromating of aluminium and aluminium alloys Organic solvents Oegreasing Aqueous acid Aqueous alkaline Aqueous organic solution solution solution J 1 i Cold water rinse 1 Pickling (de-oxidizing, matt-pickling, burnishing) (alkaline or acid) Cold water rinse I I Brightening (nitric acid plus hydrofluoric acid) Cold water rinse I Chromating Green I Transparent-Yellow-Green I Cold water rinse I Hot water rinse (below 60” 0 V (below 65 0 (100” c to 110 C) V Drying I (below 85” 0 I Lacquering I Rinsing must be performed carefully in order to avoid splattering of chemicals and corrosion damage. 7 -,-,- IS0 10546:1993(E) UDC 621.794.6:669.716.9 Descriptors: aluminium, aluminium alloys, coatings, non-metallic coatings, conversion coatings, chromate coatings, classification, specifications, tests. Price based on 7 pages