Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1999-07-22
2001-01-30
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S262000
Reexamination Certificate
active
06179934
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a treatment composition, more particularly a liquid solution in water, that forms a zinc phosphate-type conversion coating on metal surfaces and to a method for treating metal surfaces with such a treatment composition. The invention is particularly effective when applied to iron and steel, but in addition it can also be applied to a variety of surfaces that are constituted of zinc or an alloy thereof and/or aluminum or an alloy thereof, such alloys containing at least 55 atomic percent of zinc or of aluminum.
DESCRIPTION OF RELATED ART
Zinc phosphate treatments are carried out in order to impart corrosion resistance to metals such as iron and steel, as an undercoating for painting operations, and to provide lubrication in forging and wire drawing operations. This treatment is carried out by bringing the workpiece into contact with the treatment composition, often called a “bath” hereinafter, even though not necessarily contacted by immersion of the metal substrates in it, for a sufficient period of time at an appropriate temperature. Spraying and/or dipping are typically employed to effect this contact. The overall process generally proceeds through the following steps.
(1) Cleaning
(2) Surface conditioning
(3) Phosphate conversion treatment
(4) Post-treatment
A water rinse is typically used between each of these steps and the next successive one of these steps, except between steps (2) and (3), in order to avoid dragging material from an upstream step into a downstream step.
Cleaning step (1) is carried out in order to remove grease and other contaminants adhering to the workpiece and thereby provide for the smooth execution of the downstream steps. An alkaline cleaner is typically used in this cleaning step.
In the surface conditioning step (2), the workpiece is typically brought into contact with a treatment bath that contains colloidal titanium. This treatment results in a substantial acceleration of the film-forming reactions during the phosphate conversion treatment and in this manner makes possible the formation of a uniform, fine, and dense conversion coating in a short period of time. The use of phosphate coatings as paint-base coatings on automotive components requires high-quality phosphate coatings that must exhibit paint adherence and corrosion resistance, and the surface conditioning step has been considered essential for the production of coatings of this type.
Phosphate conversion treatment step (3) is run using a variety of treatment bath compositions and treatment conditions whose particular selection will depend on the specific objective. As an example, in the case of paint-base coating treatments for automotive components, the ingredients present in a zinc phosphate-type treatment bath will be phosphate ions, zinc ions, and other metal ions (e.g., nickel, manganese), and each of these components will provide specific properties to the final film. Other components typically present in baths of this type are nitrate, nitrite, chlorate, fluoroborate, and fluorosilicate. A lowering of the treatment temperature has been pursued over the last few years, and at present these treatments are run at temperatures of 40° C. to 50° C. for treatment times of about 1.5 to 3 minutes. The treatment is often carried out by dipping.
Post-treatment step (4) is carried out, for example, in order to improve the corrosion resistance and paint adherence. A treatment bath, either containing hexavalent chromium or chromium-free, is used as the post-treatment agent. This step may be omitted depending on the particular objective or application.
The high-quality zinc phosphate-type conversion films used as paint-base coatings on iron and steel can be evaluated through (1) the appearance of the conversion coating, (2) the coating weight, and (3) the P/(P+H) ratio as defined below.
The following characteristics are essential, or if so stated, preferred, for rating as “good” the appearance of a conversion coating: The conversion coating must be free of defects such as rust, blue color, and thin or incomplete coverage, and in addition is preferably a columnar and/or nodular crystalline coating whose crystals are preferably microcrystalline, with sizes from about 1 to 5 micrometers (hereinafter usually abbreviated as “&mgr;m”).
The mass of the coating formed divided by the area of the surface being coated, a value also denoted as “coating weight” hereinafter, as a general rule is preferably from about 1 to 3 grams per square meter (hereinafter usually abbreviated as “g/m
2
”).
The P/(P+H) ratio is calculated from the immediately following equation:
P/
(
P+H
) ratio=
lp/
(
lp+lh
)
where lh represents the X-ray diffraction intensity from the (020) surface of hopeite (zinc phosphate) and lp represents the X-ray diffraction intensity from the (100) surface of phosphophyllite (zinc iron phosphate) or one of its analogs in which manganese, nickel, cobalt, calcium, magnesium, copper and/or the like cations that were dissolved in the phosphating solution can replace some or all of the iron cations in phosphophyllite itself. The P/(P+H) ratio is widely recognized as a characterizing value for the zinc phosphate-type films used as paint-base coatings on iron and steel. P/(P+H) values of 0.8 to 1 are generally considered to provide good conversion coatings for paint-base coatings.
A coating solution taught in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 1-123080 (123,080/1989) uses hydroxylamine sulfate (HAS) as an accelerator. The intent in this case is through the use of this accelerator to change the film morphology from platelet structures to columnar and/or nodular structures over a broad range of zinc concentrations. While Ni and Mn are mentioned as general supplementary ions, neither the Detailed Description nor the Examples provide an explanation of how their quantity of addition should be determined. This reference also describes a relatively large number of other patents in which hydroxylamine sulfate is added to zinc phosphate solutions and in addition describes patents in which oxidizing agent is present, including U.S. Pat. Nos. 2,743,204 and 2,298,280.
Japanese Patent Application Laid Open Number Hei 1-123080 also teaches treatment of the metal surface with a colloidal titanium-containing surface conditioner prior to conversion treatment.
The hydroxylamine source is taught to be added to the conversion bath taught in Japanese Patent Application Laid Open Number Hei 1-123080 in order to broaden the permissible range of zinc concentrations in the bath at which the desired conversion coatings can be obtained. In this reference the zinc concentration range is expressed by the zinc/phosphate ions ratio. The zinc/phosphate ions weight ratio is no more than 0.27 while the concentration of the zinc itself is from 0.02 to 0.2 weight %, which is equivalent to about 0.2 to 2 grams per liter of total composition (hereinafter usually abbreviated as “g/L”).
A phosphate conversion treatment bath taught in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 5-195245 (195,245/1993)contains hydroxylamine (HAS), nickel ions, and manganese ions. This reference also teaches surface conditioning with a colloidal titanium-containing bath prior to conversion treatment.
Japanese Patent Application Laid Open Number Hei 5-195245 teaches nickel ions and manganese ions as essential components in its treatment bath and also stipulates about {fraction (1/25)} to {fraction (1/10)} as a more desirable range for the zinc ions/phosphate ions weight ratio for the treatment bath of Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 1-123080. The nickel ions content in this case is specified as from 0.02 to 0.15 weight % and the manganese ions content is specified as from 0.02 to 0.15 weight %. Also specified is a zinc ions/(manganese ions+nickel ions) weight ratio of about 1/1.5 to 1/0.5.
The phosphate conversion treatment bath taught in Japanese Patent Application Lai
Kawakami Masahiko
Kobayashi Naoyuki
Oyama Kazuyuki
Harper Stephen D.
Henkel Corporation
Jaeschke Wayne C.
Oltmans Andrew L.
Sheehan John
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