Non-chromium passivation method and composition for galvanized metal surfaces

A substantially chromium free composition and method for passivating a galvanized metal surface is disclosed. The passivation treatment which may be rinsed or dried-in-place includes phosphoric acid, boric acid and optionally molybdic acid.

FIELD OF THE INVENTION 
The present invention relates to a composition and method for passivating a 
galvanized coating on a metal substrate. More particularly, the present 
invention relates to a treatment of a galvanized or Galvalume (trademark 
of Bethlehem Steel Corporation) metal surface to inhibit corrosion without 
painting. 
BACKGROUND OF THE INVENTION 
The purposes of a formation of a chromate conversion coating on the surface 
of galvanized metal are to provide corrosion resistance, improve adhesion 
of coatings and for aesthetic reasons. Chromate passivation of a 
galvanized steel surface is done to provide corrosion resistance and for 
aesthetic reasons on materials which are not to be painted. A bulky, white 
corrosion product may form on an unprotected bright zinc surface when it 
becomes wet. This corrosion product is a mixture of zinc carbonate and 
zinc oxide or hydroxides resulting from zinc oxidation. The conditions 
producing the "humid storage" stain (so called white rust) most frequently 
occur in shipment and during storage, especially when daily temperature 
variations cause atmospheric water vapor to condense on a zinc surface. 
Likewise, black stains form on unprotected Galvalume. Galvalume is a 
trademark of the Bethlehem Steel Corporation for a zinc-aluminum 
galvanized coating over steel. 
Chrome based passivation treatments are applied to galvanized metals and 
Galvalume to provide both long term and short term corrosion protection. A 
chromate treatment is typically provided by contacting galvanized metal 
with an aqueous composition containing hexavalent and trivalent chromium 
ions, phosphate ions and fluoride ions. Growing concerns exist regarding 
the pollution effects of the chromate and phosphates discharged into 
rivers and waterways by such processes. Because of the high solubility and 
the strongly oxidizing character of hexavalent chromium ions, conventional 
chromate processes require extensive waste treatment procedures to control 
their discharge. In addition, the disposal of the solid sludge from such 
waste treatment procedures is a significant problem. 
Attempts have been made to produce an acceptable chromate free conversion 
coating for passivation of galvanized metal. Chromate free pretreatments 
based upon complex fluoacids and salts or metals such as cobalt and nickel 
are known in the art. U.S. Pat. No. 3,468,724 which issued to Reinhold 
discloses a composition for coating ferriferous and zinc metal which 
comprises a metal such as nickel or cobalt and an acid anion selected from 
the group sulfate, chloride, sulfonate, citrate, lactate, acetate and 
glycolate at a pH of from 0.1 to 4. 
SUMMARY OF THE INVENTION 
The present invention comprises a composition and method for treating the 
surface of galvanized metal to provide for the passivation of the metal 
surface. The coating formed by the present invention may be dried in place 
or rinsed. The method of the present invention comprises treating a 
galvanized metal surface with an aqueous treatment solution including 
phosphoric acid, boric acid, and optionally molybdic acid. The treatment 
solution is substantially free of chromium. By substantially free of 
chromium it is meant that no chromium is added to the system. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present inventors have discovered that a galvanized coating on metal 
can be passivated so as to provide corrosion resistance with a 
substantially chromium free treatment solution. As used herein, galvanized 
includes Galvalume.RTM. a trademark of Bethlehem Steel for zinc-aluminum 
galvanized metal. 
The treatment solution of the present invention is an aqueous solution 
including phosphoric acid, boric acid and optionally molybdic acid. The 
treatment solution may be applied by any convenient means including 
spraying, dip-squeegeeing, flow coating, and roll coating. 
The concentration ratios of the three components can vary depending upon 
the metal substrate and treatment requirements. The phosphoric acid 
concentration can range from about 0.5 to 50%, the boric acid 
concentration can range from about 0.1 to 5% and the molybdic acid 
concentration can range from 0 to about 0.5%. The preferred concentrations 
of each component are 2.0% of 75% phosphoric acid, 0.2% boric acid and 
0.2% molybdic acid. It was found that baking or oiling treated metal 
articles can increase the passivity. 
Baking is a process familiar to those skilled in the art wherein treated 
metals are heated to specific peak metal temperature. Oiling is the 
application of a protective oil coating to the treated metal surface to 
further control corrosion. 
A typical treatment process employing the treatment solution of the present 
invention can include: cleaning the unpassivated galvanized metal or 
Galvalume with an alkaline or weak acid cleaner followed by ambient tap 
water rinsing, squeegeeing and applying the treatment solution at room 
temperatures. The cleaning and rinsing stages prior to treatment solution 
application may not be necessary if the metal surface is not heavily 
soiled.

The invention will now be further described with reference to a number of 
specific examples which are to be regarded as solely illustrative, and not 
as restricting the scope of the invention. 
EXAMPLES 
The treatment solution of the present invention was tested on hot dipped 
galvanized metal and Galvalume. Comparative tests were run with a 
commercial passivation treatment, Betz Permatreat 2510 available from Betz 
Laboratories, Inc., of Trevose, PA. Betz Permatreat 2510 includes 
hexavalent chromium, phosphoric acid and trivalent chromium. 
The evaluation of the passivation treatment on galvanized metal and 
Galvalume was made through a series of tests known to one skilled in the 
art. A beaker condensation test was used which measures the tendency for 
white rust or discoloration to develop on vapor exposed metal surfaces. 
The less area where white rust or discoloration develops the better the 
passivation. In the test, the metal surface to be tested is placed over a 
600 milliliter beaker without a spout. The beaker contains warm 
(49.degree. to 54.degree. C.) water to within 1/2" at the top. The beaker 
is allowed to cool for 24 hours (1 cycle). The test panel is removed and 
inspected for corrosion or discoloration. The test is repeated by adding 
fresh warm water to the beaker and repeating the 24 hour cycle. "Stack 
testing" was employed which measures the tendency for white rust or 
discoloration to develop on wet packed metal surfaces. The results are 
checked every five day cycle. "Water immersion" testing was employed which 
measures the tendency for white rust or discoloration to develop on a 
metal surface immersed in deionized water at 49.degree. C. 
Beaker testing with Betz Permatreat 2510 resulted in no white rust or black 
stain formation after more than 10 cycles. 
EXAMPLE 1 
A series of solutions containing phosphoric acid, boric acid and molybdic 
acid were used to passivate Advanced Coating Technology (ACT) G-90 hot 
dipped galvanized metal. After cleaning with an alkaline cleaner at 
55.degree. C. for 10 seconds, rinsing with ambient tap water, squeegeeing 
and application of the treatment solution in a spin coater, the metal test 
panels were baked to peak metal temperatures of 230.degree. C. and then 
cooled in air. The test panels were then subjected to the beaker 
condensation test described above. Table I summarizes the results. 
TABLE I 
__________________________________________________________________________ 
Passivation Results 
TREATMENT BEAKER CONDENSATION* 
H.sub.3 PO.sub.4 
Molybdic 
Borax 
1st 
4th 
7th 
10th 
16th 
19th 
21st 
24th 
(75%) (%) 
Acid (%) 
(%) (cycle) 
__________________________________________________________________________ 
1.0 0.1 0.1 0 0 2 5 15 25 30 40 
1.0 0.1 0.5 0 5 6 7 19 19 19 15 
2.0 0.0 0.2 0 3 3 3 3 3 3 3 
2.0 0.0 1.0 0 5 5 10 15 20 20 25 
2.0 0.2 0.2 0 0 0 0 0 0 0 0 
2.0 0.2 1.0 0 0 3 3 10 10 10 10 
Oiled after Passivation** 
1.0 0.1 0.1 0 0 0 0 1 2 2 2 
1.0 0.1 0.5 0 0 0 1 3 3 3 3 
2.0 0.2 0.2 0 0 0 0 0 0 0 0 
__________________________________________________________________________ 
*Rust area coverages are shown in the table. The numbers are in 
percentage. 
**Castrol 924 HF oil was applied using drawdown bar #5. 
EXAMPLE 2 
The solutions described above in Table I were employed to treat 
nonchemically treated Galvalume test panels from National Steel. The 
preparation in testing methods were as described above. Table II 
summarizes the results. 
TABLE II 
__________________________________________________________________________ 
Beaker Condensation Test 
TREATMENT BEAKER CONDENSATION* 
H.sub.3 PO.sub.4 
Molybdic 
Borax 1st 7th 
8th 
9th 
10th 
12th 
15th 
(75%) (%) 
Acid (%) 
(%) (cycle) 
__________________________________________________________________________ 
1.0 0.1 0.1 0 0 0 0 0 0 0 
1.0 0.1 0.1 0 5 10 10 10 15 20 
1.0 0.1 0.5 0 2 5 5 5 20 20 
2.0 0.2 0.2 0 0 0 0 0 0 0 
Clean only 15 
100 -- 
-- -- -- -- 
0.5% PT 2510 0 0 0 0 0 0 0 
1.0% PT 2510 0 1 1 1 1 0 0 
__________________________________________________________________________ 
EXAMPLE 3 
Stack and water immersion test as described above were performed on 
nonchemically treated Galvalume test panels from National Steel prepared 
in accordance with the description of Example 1. Table III summarizes the 
results. 
TABLE III 
__________________________________________________________________________ 
Stack and Water Immersion Tests 
TREATMENT BLACK STAIN AREA (%) 
H.sub.3 PO.sub.4 
Molybdic 
Borax STACK IMMERSION 
(75%) (%) 
Acid (%) 
(%) BAKED* 
OILED** 
(5 cycles) 
(700 hrs) 
__________________________________________________________________________ 
2.0 0.2 0.2 No No 0 3 
2.0 0.2 0.2 No Yes 2 0 
2.0 0.2 0.2 Yes No 5 0 
2.0 0.2 0.2 Yes Yes 0 0 
1.0% PT 2510 No No 0 0 
1.0% PT 2510 No Yes 0 0 
1.0% PT 2510 Yes No 5 5 
1.0% PT 2510 Yes Yes 0 0 
__________________________________________________________________________ 
*Peak metal temperature: 232.degree. C. 
**Castrol 924 HF oil, applied using drawdown bar #5 
EXAMPLE 4 
An outdoor exposure test was conducted on phosphoric acid/molybdic 
acid/borax passivated ACT G-90 galvanized metal test panels. The exposure 
was for a period of approximately 7 weeks during a relatively humid spring 
season. Panels treated with Betz Permatreat 2510 were tested side by side. 
The panel preparation was as described above in Example 1. The treatment 
solution consisted of 0.6% phosphoric acid, 0.2% molybdic acid, and 0.4% 
borax. Panels treated with Permatreat 2510 and the treatment solution of 
the present invention exhibited a similar appearance at the end of the 
seven week test period. 
The results of Examples i through 4 show that the non-chromium treatment 
solution of the present invention provides passivation of galvanized metal 
and Galvalume comparable to a commercial chromium based passivation 
treatment. 
While this invention has been described with respect to particular 
embodiments thereof, it is apparent that numerous other forms and 
modifications of this invention will be obvious to those skilled in the 
art. The appended claims and this invention generally should be construed 
to cover all such obvious forms and modifications which are within the 
true spirit and scope of the present invention.