Trivalent chromium conversion coating and method of application thereof

Inactive Publication Date: 2006-11-30
PAVCO INC
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0009] The present invention provides a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The invention also provides methods for applying a conversion coating to such substrates, and coated articles prepared according to the method. The trivalent chromium coating composition is particularly useful in that it is free of hexavalent chromium and is also free of components that may facilitate conversion of trivalent chromium to hexavalent chromium. Further, the trivalent chromium coating composition is beneficial in that it provides improved corrosion protection, particularly extending the time to which a substrate having a conversion coating according to the invention can be exposed to a corrosive environment without substrate corrosion occurring.
[0011] Preferably, the conversion coating composition of the invention is free of components that may reduce the corrosion resistance properties of the conversion coating formed of the inventive composition. For example, in one embodiment, the conversion coating composition particularly avoids the use of components that may provide free fluorides. In another embodiment of the invention, the conversion coating is substantially free of chelators, in particular, carboxylic acid based chelators (such as di-oic acids).
[0013] The conversion coating composition of the invention is particularly useful in that it can be prepared as a concentrate. The concentrate provides ease of storage and transportation, and it can easily be diluted at the time of use to provide the conversion coating composition of the invention.

Problems solved by technology

Heavier chromate conversion coatings are considerably more protective than the bright finishes, but they do not meet the aesthetic criteria that are characteristic of the bright coatings.
This makes such chromate conversion coatings unfavorable since chromium (VI) is generally recognized as being toxic.
Given these findings, the waste from a hexavalent chromium based solution creates significant environmental concerns, and hexavalent chromium baths require special treatment prior to disposal.
Other types of treatments for forming passivation coatings that are non-chromium containing are known in the art, but these are generally unsatisfactory for enhancing the corrosion resistance of a plated substrate, particularly when the treated substrate is subjected to a humid environment.
Phosphate coatings are also not bright in appearance, but provide dull, opaque, paint-like overcoats lacking the aesthetic attributes of hexavalent chromium conversion coatings.
The presence of this hexavalent chromium may then lead to some or all of the previously described health and handling problems.
Furthermore, the inclusion of an oxidizing agent may add additional costs and steps to the formation of the conversion coatings.

Method used

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Examples

Experimental program
Comparison scheme
Effect test

example 1

[0058] A series of test parts were produced to evaluate various chromate formulations. The parts were plated in acid-chloride zinc electroplating consisting of 3 oz / gal zinc metal, 18 oz / gal ammonium chloride, 4% (vol / vol) Smart Zinc Carrier (available from Pavco, Inc.), and 0.2% (vol / vol) Smart Zinc Toner (available from Pavco, Inc.). The parts were zinc plated at 20 amps / ft2 for 25-30 minutes to produce a zinc thickness of approximately 0.3 to 0.5 mm. After plating, the parts were rinsed in water and then rinsed in 0.5% (vol / vol) nitric acid solution.

[0059] Various different conversion coating compositions were applied to the plated parts prepared as described above. Application was by dipping in a bath comprising a conversion coating composition according to the invention at a temperature of 50° C. and a pH of 2.6 for 60 seconds. The parts with the conversion coating composition applied thereto were tested for corrosion resistance according to ASTM B 117-03, as described above. ...

example 2

[0061] Various test parts were prepared as described in Example 1 and were coated with further conversion coating compositions according to the present invention to evaluate the ability of the coatings to resist corrosion. Application was by dipping in a bath comprising the conversion coating composition a temperature of 40 or 50° C. and a pH of 2.6 for a time of 60 seconds. The parts with the conversion coating composition applied thereto were tested for corrosion resistance according to ASTM B 117-03, as described above. Conversion coating composition ingredients (in moles / L), mole ratio of nitrate ions to combined chromium (III) and cobalt (II) ions, and time to white salts (in hours) are provided below in Table 2.

TABLE 2Mole RatioTime to[Cr+3][Co+2][NO3−][SO4−2]NO3− / whiteRun(M)(M)(M)(M)(Cr+3 + Co+2)salts (hrs)17a0.2600.0271.2000.6604.230318b0.2000.0401.2060.0405.047718c0.2000.0401.2060.0405.0608200.2000.0401.0000.3404.2317210.2000.0400.9000.5203.8272220.2000.0201.2000.2605.530...

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Abstract

The present invention is directed to a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The conversion coating composition comprises chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. The invention further provides methods of applying a conversion coating to a plated article and articles with a conversion coating applied thereto.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. Provisional Application No. 60 / 684,939, filed May 26, 2005, which is incorporated herein in its entirety.FIELD OF THE INVENTION [0002] The present invention is generally related to the treatment of a metal surface for improving the properties thereof, particularly the corrosion resistance of the metal surface. More specifically, the invention is related to conversion coatings, preferably coatings useful for applying to a metal plated surface. BACKGROUND [0003] Conversion coatings are commonly used in the metal finishing industry to provide improved properties to metal surfaces, particularly metal plated surfaces. Conversion coatings are generally known to be particularly useful for providing improved corrosion resistance to the metal and also for improving adhesion properties of the metal in relation to additional coatings that may later be added, such as paint or other finishes. Multiple types...

Claims

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Application Information

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IPC IPC(8): C23C22/53
CPCC23C22/34C23C2222/10C23C22/53
Inventor DIADDARIO, LEONARD L. JR.STORER, GREGORY E.PROPER, BRADLEY J.
Owner PAVCO INC
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