Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1999-09-03
2001-04-10
Jones, Deborah (Department: 1775)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S259000, C106S014120, C106S014440
Reexamination Certificate
active
06214132
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a surface conditioning pretreatment bath and surface conditioning process for use prior to the phosphate conversion coating treatments that are executed on the surfaces of metals such as iron and steel, zinc-plated steel sheet, aluminum, and the like. The subject surface conditioning pretreatment bath and process have the effect of accelerating the conversion reactions and shortening the reaction time in the ensuing conversion treatment, while also producing finer crystals in the phosphate coating.
DESCRIPTION OF RELATED ART
The formation of fine-sized, dense phosphate coating crystals on metal surfaces is currently required in the field of automotive phosphate treatments in order to improve the post-painting corrosion resistance and in the field of phosphate treatments for cold-working applications in order to reduce the friction during working such as drawing and extend the life of the working tools. This requirement has led to the execution of a surface conditioning step prior to the phosphate conversion coating treatment. The purpose of the surface conditioning step is to activate the metal surface and produce nuclei for deposition of the phosphate coating crystals in order to ultimately produce fine-sized, dense crystals in the phosphate coating. A typical phosphate conversion coating process that produces fine-sized, dense phosphate coating crystals can be exemplified as having the following steps:
(1) Degreasing and/or other cleaning
(2) Tap water rinse (often multistep)
(3) Surface conditioning
(4) Phosphate conversion coating treatment
(5) Tap water rinse (often multistep)
(6) Rinse with pure water.
The surface conditioning step is carried out in order to render the phosphate coating crystals fine-size and dense. Compositions for this purpose are known, for example, from U.S. Pat. Nos. 2,874,081, 2,322,349, and 2,310,239. Disclosed therein as the main constituents of the surface conditioner are titanium, pyrophosphate ions, orthophosphate ions, sodium ions, and the like. These surface conditioning compositions, known as Jernstedt salts, provide titanium ions and colloidal titanium in their aqueous solutions. The colloidal titanium becomes adsorbed to the metal surface when the degreased and water-rinsed metal is dipped in an aqueous solution of such a surface conditioning composition or when the metal is sprayed with the surface conditioning pretreatment bath. The adsorbed colloidal titanium functions in the ensuing phosphate conversion coating treatment step as nuclei for deposition of the phosphate coating crystals, thereby accelerating the conversion reactions and causing the phosphate coating crystals to be finer-sized and denser. The surface conditioning compositions in current industrial use all employ Jernstedt salts. However, the use in the surface conditioning step of colloidal titanium generated from Jernstedt salts is associated with a variety of problems.
The first problem is a deterioration with time in the surface conditioning pretreatment bath. While the heretofore employed surface conditioning compositions do provide remarkable fine-sizing and densifying effects on the phosphate coating crystals immediately after preparation of the aqueous solution of the composition, this activity can be lost several days after preparation because of aggregation of the colloidal titanium. This loss in activity, which manifests as a coarsening of the phosphate coating crystals, occurs regardless of whether the surface conditioning pretreatment bath has actually been used during this several day period. To respond to this problem, Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 63-76883 (76,883/1988) teaches a process for managing and maintaining the surface conditioning activity by measuring the average particle size of the colloidal titanium in the surface conditioning pretreatment bath and continuously discarding bath so as to keep the average particle size below a prescribed value. Fresh surface conditioning composition is also supplied to make up for the discarded portion. This method does permit a quantitative management of the factors related to the activity of the surface conditioning pretreatment bath, but at the same time this method requires that large amounts of the surface conditioning pretreatment bath be discarded in order to maintain an activity level equal to that of the initially prepared aqueous solution. This creates an additional problem with respect to the waste water treatment capacity of the plant where the process is carried out. In sum, the activity is maintained by the combination of continuously discarding the surface conditioning pretreatment bath and make up of the entire quantity.
The second problem is that the activity and life of the surface conditioning pretreatment bath are substantially affected by the quality of the water used for bath buildup. Industrial-grade water is generally used to make up the surface conditioning pretreatment bath. However, as is well known, industrial-grade water contains cationic components which are a source of total hardness, e.g., magnesium and calcium, and the content of these components varies as a function of the source of the industrial-grade water used for bath buildup. It is also known that the colloidal titanium, which is the principal component of the heretofore used surface conditioning pretreatment baths, bears an anionic charge in aqueous solution and that the resulting mutual electrical repulsion prevents its sedimentation and supports the maintenance of its disperse state.
As a consequence, the presence of large amounts of cationic calcium or magnesium in the industrial-grade water causes electrical neutralization of the colloidal titanium. This in turn causes a loss of the repulsive force between the particles of dispersed titanium colloid, which results in aggregation and sedimentation and hence in a loss of activity. The addition of a condensed phosphate such as pyrophosphate to the surface conditioning pretreatment bath has been proposed for the purpose of blocking the cationic component and maintaining the stability of the colloidal titanium. However, when condensed phosphate is added in large amounts to a surface conditioning pretreatment bath, the condensed phosphate reacts with the surface of steel sheet with the formation thereon of an inert coating and in this manner causes conversion coating defects in the ensuing phosphate conversion coating treatment step. At locations where the calcium and magnesium content is very high, pure water must be used for buildup of the surface conditioning pretreatment bath and for feed to the bath; this is a major economic drawback.
Restrictions on temperature and pH during use of prior art colloidal titanium conditioning treatments are a third problem. In specific terms, at a temperature above 35° C. or a pH outside the range from 8.0 to 9.5, colloidal titanium usually undergoes aggregation and cannot exhibit its surface conditioning activity. The prior art surface conditioning compositions must therefore be used at a prescribed temperature and pH range. It is also not possible to generate a long-term cleaning and activating activity for metal surfaces using a single liquid comprising the combination of surface conditioning composition with degreaser, etc.
A fourth problem concerns the limitation on the degree of fine-sizing of the phosphate coating crystals that can be achieved through the activity of the surface conditioning pretreatment bath. The surface conditioning activity is generated by the adsorption of colloidal titanium on the metal surface, which creates nuclei for the deposition of the phosphate coating crystals. As a result, the phosphate coating crystals become denser and finer as the number of colloidal titanium particles adsorbed on the metal surface during the surface conditioning step increases. This would upon initial analysis lead to the idea of increasing the number of colloidal titanium particles in the surface conditioning pretreatment bath, i.e., increa
Nagashima Yasuhiko
Nakayama Takaomi
Shimoda Kensuke
Harper Stephen D.
Henkel Corporation
Jaeschke Wayne C.
Jones Deborah
Koehler Robert R.
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