Coating processes – With post-treatment of coating or coating material – Liquid extraction of coating constituent or cleaning coating
Patent
1993-12-27
1996-03-12
Utech, Benjamin
Coating processes
With post-treatment of coating or coating material
Liquid extraction of coating constituent or cleaning coating
427354, 427421, 427427, B32B 712
Patent
active
054987598
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a novel method for rapidly treating the surface of aluminum and aluminum alloy (hereinafter jointly described simply as "aluminum", unless the context implies otherwise), a method which rapidly produces on the aluminum a conversion film having a chromium add-on equivalent to the chromium add-on of films produced by prior phosphoric acid-based chromating treatments and which provides the aluminum surface with an excellent corrosion resistance and paint adherence. The present invention is particularly well suited for treating the surface of aluminum coil.
BACKGROUND ART
Conversion treatments which provide the surface of aluminum with an excellent corrosion resistance and paint adherence are typified by chromic acid-based chromating treatments and phosphoric acid-based chromating treatments. The conversion coatings produced by chromic acid-based chromating treatments (Japanese Patent Publication Number 45-41088 [41,088/70] and British Patent Number 1,087,757) contain hexavalent chromium and therefore have a yellow or gold color. Thus, the color of this film will remain visible when a clear coat is applied on the surface. Moreover, since the conversion coating contains toxic hexavalent chromium, the development of a Cr(VI)-free conversion coating is desired.
In contrast to the preceding, the coatings formed by phosphoric acid-based chromating treatments do not contain hexavalent chromium and thus are colorless. As a result, the use of this treatment has been spreading in recent years. The phosphoric acid-based chromating treatment disclosed in U.S. Pat. No. 2,438,877 employs a treatment bath of chromic acid (CrO.sub.3), orthophosphoric acid (H.sub.3 PO.sub.4), and hydrofluoric acid (HF), and the principal component of the resulting film is chromic phosphate hydrate (CrPO.sub.4 .multidot.4H.sub.2 O). When this particular phosphoric acid-based chromating treatment is run in an aluminum coil line using the older conversion treatment and times of 5 to 10 seconds, a strongly corrosion-resistant, highly paint-adherent conversion coating having a chromium add-on of 20 to 50 mg/m.sup.2 is produced thereby.
Also within the sphere of Cr(VI)-free conversion treatment baths, Japanese Patent Application Laid Open [Kokai or Unexamined] Number 63-86875 [86,875/88] teaches the addition of molybdate to a zinc phosphate conversion treatment bath. However, the purpose of this conversion treatment bath is to passivate the surface of composite materials fabricated from different types of metals, and it also contains trivalent chromium, zinc oxide, and acetate as essential components in addition to molybdate.
DISCLOSURE OF THE INVENTION
On the subject of the aforementioned phosphoric acid-based chromating treatments, aluminum coil lines have been operated at increasing speeds in recent years in order to improve their productivity, and conversion treatment times have been shortened to 1 to 3 seconds. However, a highly corrosion-resistant, highly paint-adherent conversion film having a chromium add-on .gtoreq.20 mg/m.sup.2 essentially cannot be produced when prior phosphoric acid-based chromating treatments are installed in these accelerated lines, thus creating a problem which largely cannot be addressed by prior phosphoric acid-based chromating treatments. Accordingly, the development of a phosphoric acid-based chromating treatment which can generate a conversion coating having a chromium add-on .gtoreq.20 mg/m.sup.2 at conversion treatment times .ltoreq.3 seconds is desired.
The above-described problem was carefully examined, and it was discovered as a result that a conversion film with an unusually good corrosion resistance and paint adherence can be formed on the surface of aluminum by spraying same for 0.5 to 3 seconds with a chromating bath (heated to 40.degree. to 60.degree. C.) which has a pH of 1.0 to 3.0 and which contains 0.1 to 3.0 g/L of ions selected from tungstate ion and/or molybdate ions, 0.5 to 4.0 g/L of hexavalent chromium ions, 5.0 to 30.0 g/L of phosphate
REFERENCES:
patent: 2438877 (1948-03-01), Spruance
patent: 4775427 (1988-10-01), Portz et al.
patent: 4812175 (1987-03-01), Reghi
Nakada Kazuya
Yoshida Masayuki
Henkel Corporation
Jaeschke Wayne C.
Robbins Beatrice N.
Utech Benjamin
Wisdom, Jr. Norvell E.
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