We send our coupons out for testing to the hours salt spray per the standard. We also perform the wet tape testing per the standard. Does this also qualify us to do class 1A finishing? We have been subcontracting class 1A finishes, but reading the standard, I see no difference other than the conductivity for which there is no testing mandated. If we do the required test to qualify us to do class 3 finishes, does this qualify us to do class 1A? NObody understands that spec!
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A number in parentheses indicates the year of last reapproval. Asuperscript epsilon indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the U. Department of Defense. No other units of measurement are included in thisstandard.
It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Referenced Documents2. NOTE 1Nonrinsed chromate coatings are finding increased usage onfabricated parts and castings. Current edition approved March 1, Published April Originallyapproved in DOI United States1protection to yellow, intermediate thickness coatings Class 2suitable as an organic film base or to colorless, thin coatingsClass 3 suitable for low electrical contact resistance.
Theyellow coatings vary from golden yellow to iridescent lightyellow. Chromate-phosphate finishes Class 4 can be appliedranging in color from green to iridescent light green. If it is necessary tohave exact shades of color, it is possible to dye chromatecoatings having a coating mass greater than 0.
It should be noted that color and coloruniformity will vary somewhat between one alloy and anotherand from a polished surface to an etched surface. Iridescenceand variations in color density from one area of the surface toanother are normal and shall not be considered a sign of poorquality.
Surfaces Preparation5. The partsmust therefore, as far as necessary, be cleaned before chromat-ing and if necessary be pickled. Methods of Application of Chromate Coatings6. The appli-cation method used should be taken from the operatinginstructions for the chromating process employed. Chromatingsolutions are usually acidic and may contain hexavalentchromium salts together with other salts that may be varied toaffect the appearance and hardness of the film. The color of thefilm, and, therefore, the type of conversion coating, depends onthe composition of the chromating solution, but it is alsoaffected by the pH and temperature, the duration of thetreatment, and the nature and surface condition of the alloybeing treated.
If hot water is used as the final rinse after thechromating process, it is essential that the time of rinsingshould be kept as short as possible in order to prevent thedissolution of the hexavalent chromium.
The drying of thecoating shall be carried out at a temperature not exceeding60C to prevent cracking due to dehydration, which causes lossof adhesion and performance of the chromate coating. Coating Requirements7. They should, therefore, be handledcarefully for the first 24 h after treatment, and any testsincluding corrosion tests shall be deferred until the expirationof that period.
The green chromate-phosphate coatings usuallycontinue to improve in corrosion resistance after initial forma-tion. They achieve their maximum corrosion resistance after 1to 2 months at room temperature. It is not required to storeparts for this purpose. When measured at9-V and a 2-A current the resistance should be less than 0. There are no practical tests for measuring theadhesion of a chromate conversion coating on aluminum.
However, a practical evaluation of the adhesion can be madeby measuring the adhesion of a secondary organic film appliedto the chromated aluminum. When specified, the chromateconversion coating shall pass the organic coating adhesion testin Test Methods D or the equivalent ISO None shall be larger than 1 mm indiameter. BThe exposure times are indicative of the relative corrosion resistance of thevarious coating classes on the different alloys, but no direct relationship existsbetween performance in the neutral salt spray test and performance in service.
The least quantity of total chromiumdetectable by this method corresponds to about 5 mg chromiumper square metre of surface. The resulting solution is filtered, and 25 mL of the filtrate areacidified with approximately 38 nitric acid then 10 mLammonium molybdate reagent and 5 g ammonium nitrate areadded.
The sample is allowed to stand for at least 15 min. Ayellow precipitate shows the presence of phosphate. The leastquantity of phosphate detectable by this process corresponds toabout 40 mg P2O5per square metre of surface. The resultingsolution is filtered through glass wool, and 25 mL of the filtrateare neutralized with 20 caustic soda until phenolphthaleinindicator gives a red coloration.
The solution is then weaklyacidified by addition of about 10 drops of 25 sulfuric acid. After addition of 5 mL of 5 potassium ferrocyanide solution,a greenish-white precipitate shows the presence of zinc. Theleast quantity of zinc detectable by this process corresponds toabout 20 mg zinc per square metre of surface. See Fig. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.
This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters.
Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below. Individual reprints single or multiple copies of this standard may be obtained by contacting ASTM at the aboveaddress or at phone, fax, or serviceastm.
More B Aluminum and aluminum alloys are chromate coated in order to retard corrosion; as a base for organic films including paints, plastics, and adhesives; and as a protective coating having a low electrical contact impedance. The materials are classified according to its coating thickness: Class 1; Class 2; Class 3; and Class 4. Chromate conversion coatings are normally applied by dipping: the coating may also be applied by inundation, spraying, roller coating, or by wipe-on techniques. This abstract is a brief summary of the referenced standard. It is informational only and not an official part of the standard; the full text of the standard itself must be referred to for its use and application.
Mil-C-5541 Class 3 and Class 1A