Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2001-01-22
2002-11-12
Nutter, Nathan M. (Department: 1711)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S014120, C106S014440, C148S256000, C148S259000, C428S472300
Reexamination Certificate
active
06478860
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
This invention generally concerns the art of phosphate conversion coating treatments that are executed on the surfaces of such metals as iron, steel, zinc-plated steel sheet, aluminum, and magnesium alloys. More specifically, the invention concerns a composition and process for conditioning metal surfaces prior to such conversion treatments in order to accelerate the conversion reactions, shorten the treatment time, and microsize the phosphate coating crystals.
The formation of dense, microfine phosphate coating crystals is considered desirable both within the realm of automotive phosphate treatments and within the realm of the phosphate treatments associated with plastic working. The formation of such a coating is considered desirable in the former case in order to improve the post-painting corrosion resistance and in the latter case in order to reduce friction during pressing and extend the life of the press tool. In order to obtain dense, microfine phosphate coating crystals, a surface conditioning process is executed prior to the phosphate conversion coating treatment, with a goal of activating the metal surface and producing nuclei for deposition of the phosphate coating crystals. The following treatment sequence is a generalized example of the phosphate conversion coating processes used to produce dense, microfine phosphate coating crystals:
(1) Degreasing;
(2) Tap water rinse (multistage);
(3) Surface conditioning;
(4) Phosphate conversion coating treatment;
(5) Tap water rinse (multistage);
(6) Purified water rinse.
The surface conditioning step is used to induce the formation of dense, microfine phosphate coating crystals. Compositions used for this purpose are known from, for example, U.S. Pat. Nos. 2,874,081, 2,322,349, and 2,310,239, in which titanium, pyrophosphate ions, orthophosphate ions, and sodium ions are disclosed as the main constituent components in the surface conditioning agent. These surface conditioning compositions, known as jernstedt salts, contain titanium ions and titanium colloid in aqueous solution.
The titanium colloid becomes adsorbed on the metal surface when the degreased, water-rinsed metal is dipped in or sprayed with an aqueous solution of the surface conditioning composition. The adsorbed titanium colloid forms nuclei for deposition of the phosphate coating crystals in the ensuing phosphate conversion coating treatment and thereby supports and induces an acceleration of the conversion reactions and a microfine-sizing and densification of the phosphate coating crystals. The surface conditioning compositions currently in commercial use all employ Jernstedt salts, but a number of problems have been associated with the use in surface conditioning processes of titanium colloids obtained from Jernstedt salts.
A first problem is the timewise deterioration in the surface conditioning liquid composition. Aqueous solutions that have just been prepared from the prior-art surface conditioning compositions are in fact very effective in terms of microfine-sizing and densification of the phosphate coating crystals. However, within several days after preparation of the aqueous solution, these baths suffer from a loss of activity due to aggregation of the titanium colloid—regardless of whether or not the surface conditioning liquid composition has been used during this period of time. This loss of activity results in a coarsening of the phosphate coating crystals.
In order to deal with this problem, Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 63-76883 (76,883/1988) has disclosed a method for maintaining and managing the surface conditioning activity. In this method, the average particle size of the titanium colloid in the surface conditioning liquid composition is measured and the surface conditioning liquid composition is continuously discharged so as to maintain the average particle size below a specific constant value. In addition, surface conditioning composition is supplied in an amount sufficient to compensate for the amount discharged. While this method does make possible a quantitative management of the primary factor related to the activity of the surface conditioning liquid composition, it also requires the discharge of surface conditioning liquid composition in order to maintain the activity. Moreover, this method requires the discharge of large amounts of surface conditioning liquid composition in order to maintain the same liquid composition activity as in the initial period after preparation of the aqueous solution. This creates issues with regard to the waste water treatment capacity of plants that employ this method, and as a result the activity is actually maintained through a combination of continuous discharge of the surface conditioning liquid composition and total renewal.
A second problem is that the activity and life of the surface conditioning liquid composition depend strongly on the quality of the water used for surface conditioning liquid composition build up. Industrial water is typically used to build up surface conditioning baths. However, as is well known, most industrial water contains cationic components, e.g., calcium and magnesium, that make the water “hard”, and the content of this component varies as a function of the source of the industrial water. It is known that the titanium colloid which is the main component of the prior-art surface conditioning baths carries an anionic charge in aqueous solution and is maintained in a nonsedimenting, dispersed state by the corresponding electrical repulsive forces. When the cationic component in industrial water is present in a large amount, the titanium colloid is electrically neutralized by the cationic component, so that the electrical repulsive forces are no longer effective and the activity of the titanium colloid is thereby nullified due to the occurrence of aggregation and sedimentation.
The addition of condensed phosphates such as pyrophosphates to surface conditioning baths has been proposed in order to sequester the cationic component and thereby maintain the stability of the titanium colloid. However, when added in large amounts to a surface conditioning liquid composition, the condensed phosphate reacts with the surface of the steel sheet to form a coating, which results in the production of conversion defects in the ensuing phosphate conversion coating treatment. Finally, in localities that suffer from very high magnesium and calcium concentrations, the surface conditioning liquid composition must be built up and supplied with water using pure water, which is very uneconomical.
A third problem involves the temperature and pH conditions that must be used during the surface conditioning process. Specifically, surface conditioning activity cannot be generated at a temperature in excess of 35° C. and a pH outside 8.0 to 9.5 due to aggregation of the titanium colloid. This has necessitated the use of very specific temperatures and pH ranges when using the prior-art surface conditioning compositions. This has also made it impossible to achieve cleaning and activation of metal surfaces on a long-term basis using a single liquid composition formulated by the addition of surface conditioning composition to a degreaser.
A fourth problem is the lower limit on the microfine-sizing of the phosphate coating crystals that can be obtained through the activity of the surface conditioning liquid composition. The surface conditioning activity is obtained by the adsorption of the titanium colloid on the metal surface to form nuclei for deposition of the phosphate coating crystals. Thus, finer, denser phosphate coating crystals will be obtained as larger numbers of colloidal titanium particles become adsorbed on the metal surface during the surface conditioning process.
From this one might at first draw the conclusion that the number of titanium colloid particles in the surface conditioning liquid composition should simply be increased, i.e., that the concentration of the titanium colloid should be raised. However, when
Bannai Hirokastu
Nagashima Yasuhiko
Nakayama Takaomi
Shimoda Kensuke
Harper Stephen D.
Henkel Corporation
Nutter Nathan M.
Seifert Arthur G.
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