JPS58177477A - Black chromating agent - Google Patents

Abstract

PURPOSE:To provide a black chromating agent of a prescribed pH consisting of prescribed percentages of chromic anhydride, sulfuric acid, phosphoric acid and silver orthophosphate and forming a film with superior wear and corrosion resistances on the surface of metallic zinc without generating interference colors. CONSTITUTION:An aqueous soln. contg. 5-50g/l chromic anhydride, 3-10g/ lsulfuric acid, 0-1g/l phosphoric acid and <=2g/l silver orthophosphate is prepared, or a mixed liq. is prepared by adding powdered zinc to the soln. by 100pts.wt. of the amount of the chromic anhydride. The soln. or liq. is adjusted to 0.5-1.5pH, and metallic zinc is immersed in the soln. or liq. to form a film with superior wear and corrosion resistances on the surface without generating interference colors.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a treatment agent for forming a black chromate film on the surface of metallic zinc or its alloy.

Regarding black chromate treatment technology, U.S. Pat. No. 2,483,510, U.S. Pat.
The specification of No. 19489 discloses the use of a treatment agent to which silver ions are added together with a K 6 fdti chromium compound, various acid groups, salts, etc., and even at present, hexavalent chromium compounds and their salts, sulfuric acid and its salts, phosphoric acid, etc. and its salts, hypophosphorous acid" 3J: All from the group of commercially available chemicals such as salts of phosphorous, organic acids such as acetic acid and their salts, and silver compounds such as silver nitrate, silver sulfate, and silver chloride.
A processing agent prepared by mixing and dissolving two parts in an appropriate composition ratio is widely used. In many cases, the above-mentioned treatment agent is actually supplied in a concentrated and highly concentrated form for reasons of workability and economy in preparing and replenishing baths.
Mainly due to the difficulty of solubilizing silver ions, we have selected two liquids: a mixed acid liquid consisting of a chromium compound and an acid that does not contain silver ions, and a liquid in which silver ions are made soluble with nitrate groups, and silver ion precipitation. It is provided using a combination of a mixed acid liquid containing silver ions and a powder of a hexavalent chromium compound with the concentration of hexavalent chromium compound suppressed to the limit.

However, with the composition of such a conventional black chromate treatment agent, as mentioned above, when the concentration is increased, it is easy to generate poorly soluble compounds, and if the chromate treatment is continued, trivalent chromium is generated in the liquid. Ions and zinc ions accumulate and age, and eventually the aging bath must be discarded and a new bath constructed, which, coupled with the need for complex bath management, poses problems in terms of workability and economy. Ta.

In consideration of the circumstances mentioned in J., orthophosphate aqueous solution is selected as the least dangerous and stable aqueous solution of high concentration silver ions, and other processing agent components are limited. This led to the completion of the present invention.

That is, the black chromate treatment agent according to the present invention contains 5 to 509 chromic anhydride/l! s Good IL<Ha lO~30
9/(1, & Hii? 3~10 f//(!z Good'!
L < u 5~'7 f/l! z 'J n(lk
wo0~l f/l! , tL<ha0.6 f/(1,
E') > Acid silver 2 f/l or less, preferably 1 to 21
It is characterized by being mixed and dissolved at a ratio of 1/(l),
More preferably, powdered zinc is added to the aqueous solution at a ratio of 5 to 30 parts by weight based on 100 small parts of chromic anhydride, and all of them are characterized by adding powdered zinc to the aqueous solution at a ratio of 5 to 30 parts by weight based on 100 small parts of chromic acid anhydride. It contains virtually no salt. In the above, silver orthophosphate is effective even at a very low concentration of -1, but when it exceeds 2 f/l, the chromate film tends to fall off, and when the amount of powdered zinc is less than 1 part, the effect of adding it is almost negligible. If the amount exceeds 30 parts by weight, problems such as graying of the chromate film will occur. In addition, it is preferable that these processing agents have a pH of 5 to 1.5.

The detailed content of the formation reaction mechanism of the black chromate film is still unknown. However, roughly speaking, the following three reactions are considered to be occurring as unreacted.

Dissolution reaction of Zn Zn + 2H"=-zr+F"10H4...direct]) Reduction reaction of Or. -10+38-oP
...(2)Ag
To the substitution reaction g + ten θ = Ag ~
13) Reactions (1) and (2), and (1) and (3)
) reactions occur in 16 columns, so each reaction (1
) is rate-limiting, the reaction rate becomes a rate-limiting step.

3 Zn + 3 H2SO4 + H2Cr2n7 =
3 ZnSO4 12 cr (oo), 1 H20 ... (4) 3 Zn + 2 Ag5PO4" zn3 (po
4) 2+6 A, g...(5) 1-(4) is 1
The chromate film formed by the reaction described in step 3 does not actually show a clear diffraction image, and as seen in the literature, it is ortho-crosslinked or OH-crosslinked, and it also contains acid radicals such as S and P4b. Anion occlusion is also said to be a re-dissolution reaction during the reaction of the film, so it is not simple. The reaction (5) above is relatively simple, but Konemo (4
), it is not clear whether or not it will clearly precipitate as metallic silver. Although there are many unknown points about black chromate treatment, as in the present invention, black chromate treatment containing silver phosphate,
It has been found through experimental facts that an excellent black chromate film can be obtained by adding powdered zinc in advance to IJII Lt (a treatment agent with a limited composition range).

The black chromate treatment agent of the present invention does not require complicated bath management such as bath pH adjustment with acid or alkali, disposal of aging solution, and rebuilding baths, which are found in conventional methods, except in special cases. It is not necessary; if you prepare a bath repeatedly, you can change the bath composition and bath activity by simply replenishing chromic anhydride, sulfuric acid, or a mixed acid of sulfuric acid and phosphoric acid, and silver orthophosphate on a regular basis. The ingredients can be maintained semi-permanently. In addition, in the case of a treatment agent in which powdered zinc is added at the time of bath preparation, it is possible to obtain a stable chromate treatment from the beginning of bath preparation, as well as to create a black chromate treatment with a deeper blackness and brightness. A film can be formed. Furthermore, since the processing agent according to the present invention does not substantially contain 4-flukaline salt as in the past, it is possible to suppress the occurrence of a characteristic reddish interference color that is thought to be caused by this component, and
A black chromate film with uniform gloss and excellent abrasion resistance and corrosion resistance can be produced in a short period of time.

Next, examples of the present invention will be shown to clarify the structure and effects thereof.

"Example II Treatment agent composition Chromic anhydride] 5f//1 Sulfuric acid 6 fj/1 'J nllv
O, 6f//1 orthosilver phosphate 1 f/l pH/1 Chromate treatment under the conditions described above for 325 seconds at immersion produces a black chromate film that has excellent corrosion resistance and almost the same blackness as conventional products. It was done.

C Example 2] Treatment agent composition Chromic anhydride 15 q/(! Sulfuric acid 6 Vl -phosphoric acid
0.69/(!Silver phosphate 1
9/e Powdered zinc 1 f/(! PH < 1 Immersion time 25 seconds Chromate treatment under the above conditions is very effective. The black chromate film has extremely good corrosion resistance and corrosion resistance.
The test was completed in 140 hours based on a salt spray test. Once broken, the film exhibited a deep jet black color reminiscent of glass, something never seen before.

[Example 3] Treatment agent composition Chromic anhydride 3 of//l Sulfuric acid 6 V/! Phosphoric acid 0.6g/l Orthosilver phosphate 2 f/1 Powdered zinc 19/l pH (: l Immersion time 25 seconds) Chromate treatment under the above conditions resulted in a jet black color with (-1 rise, corrosion resistance superior to conventional methods) The colored chromate film is great.

1 Example 4J Treatment agent composition Anhydrous chromium sulfur 2 of/(! Sulfuric acid 6 Vl Phosphorus Whitening 0,
6f/7'1iE silver phosphate], f/(
! Powdered zinc E-f/β pH 1 Immersion time 25 seconds

Claims (1)

[Claims] (1) Chromic anhydride from 5 to 50 V1% sulfuric acid from 3 to 50%
109/e, l) NG acid wo ~19/I! Sn”,
A black chromate treatment agent characterized in that phosphorus (fafAI) is mixed and dissolved at a ratio of 2 g/e or more. (3) Mix and dissolve chromic anhydride at a rate of 5 to 50 g/l, sulfuric acid at 3 to 109/l, phosphoric acid at O to 1 g/l, and silver orthophosphate at a ratio of 2 Da/l or less. A black chromate treatment agent characterized by further adding powdered merchant marine at a ratio of 5 to 30 parts per 100 parts of chromic anhydride. (4) pH HO, 5 to 1.5. A black chromate treatment agent according to claim 3.