Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1993-04-15
1994-11-08
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148255, 148261, C23C 2278
Patent
active
053623339
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for treating zinc and zinc alloy surfaces. In the description below, except where the context requires otherwise, the term "zinc" when used to describe surfaces to be treated is to be understood as including alloys that are predominantly zinc. More particularly, the present invention relates to a method for treating a surface of zinc-plated or zinc alloy-plated steel, prior to a chromating treatment, to provide corrosion resistance and/or function as a paint undercoat.
BACKGROUND ART
Sacrificial anodic protection based on zinc plating or zinc alloy plating is the most effective and most economical method for the corrosion protection of iron and steel. As a consequence, galvanized steel sheet accounts for 10 million tons or 10% of Japan's annual raw steel output of 100 million tons. Galvanized steel sheet is widely employed for building materials, automobiles, household electrical appliances, and the like.
In sacrificial anodic protection by zinc, the two metals (zinc and iron or steel) are in contact and form an electrochemical cell, and the zinc, as the baser metal, becomes the anode and renders the iron cathodic. This inhibits corrosion of the iron or steel by preventing the anodic dissolution which would occur in the case of iron by itself due to local cell formation. Accordingly, when the zinc in contact with the iron or steel has finally been consumed, the anticorrosion activity disappears. Therefore, preventing corrosion of the zinc layer itself (generally white rust corrosion) is crucial for extending the durability of galvanized steel material, and to this end galvanized steel is normally chromated as an undercoating treatment and then painted.
However, these two anticorrosion treatments (chromate treatment and painting) suffer from certain problems. Thus, while the chromate treatment of zinc-plated steel sheet or zinc alloy-plated steel sheet results in a very substantial inhibition of white rust development, this treatment can cause the development of black rust (known as "blackening") during storage or transport of the galvanized steel sheet. It has been observed that this phenomenon tends to occur more readily in the case of a skin pass-rolling after zinc plating and more easily when several % aluminum is present in the zinc than for ordinary galvanized steel sheet.
On the other hand, there has been a substantial proliferation in metal materials in the sector concerned with the production of colored galvanized steel sheet (widely employed for roofing and siding) by painting zinc-plated steel sheet and zinc alloy-plated steel sheet, particularly in coil form. Because the available surface treatments with reactive phosphate salts have not been able to respond to these developments, coating-type chromate treatments, which can be applied to many types of materials, tend to be used for surface treatment. However, adherence by the paint film is a normal problem here, and a problematic paint film adherence is associated with the bending of galvanized steel sheet and particularly with the bending of ultralow-lead galvanized steel sheet and zinc/aluminum alloy-plated steel sheet.
Flash treatment with, e.g., Ni, Co, Fe, etc., as described, e.g., in Japanese Patent Publication Numbers 52-22,618 [22,618/77]and 52-43,171 [43,171/77]and Japanese Patent Application Laid Open [Kokai or Unexamined]Number 59-177,381 [177,381/84]) is considered at present to be a powerful countermeasure against both blackening after chromating and low paint adherence of painted galvanized steel sheet. Japanese Patent Publication Numbers 52-22,618 and 52-43,171 teach that an improved paint adherence is obtained through a substitutional plating treatment (executed prior to chromating) with an acidic solution (pH about 1.5) containing, e g., Ni.sup.2+, Co.sup.2+, Fe.sup.2+, Fe.sup.3+. In the examples provided therein, the substitutional plating treatment is immediately followed by a water rinse.
Japanese Patent Application Laid Open Number 59-177,381 teaches that blackeni
Ishii Hitoshi
Mizuno Kensuke
Yoshitake Noriaki
Henkel Corporation
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Novell E.
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