1. Field of the Invention
This invention relates to both electrolytic and immersion plating of aluminum and particularly relates to the stannate process for plating aluminum with tin.
2. Review of the Prior Art
Aluminum is an electrochemically very active metal and readily combines with any oxygen present to form a compact, continuous coating of Al.sub.2 O.sub.3. Bare aluminum surfaces consequently do not exist except in an absolute vacuum so that when an aluminum object is looked at or touched, the surface perceived is not aluminum but aluminum oxide. This ever-present aluminum oxide film, 10-150 Angstroms thick, is largely responsible for aluminum's excellent corrosion resistance, protecting the bulk aluminum from the environment with a relatively inert coating that is self-healing if disturbed. However, this oxide coating is also largely responsible for the difficulty in achieving adherent electrodeposits because it prevents intimate interatomic contact between the aluminum and the plated metal.
Any plating process for aluminum must substantially remove the oxide film or otherwise negate its normally negative effect on adhesion. It is true that other factors also may play a role in adhesion, such factors being aluminum's relative position in the electromotive series with respect to the plating bath metal and the respective crystal structures, atomic diameters, and mutual solubilities of the metals, but the removal of the oxide coating is of foremost importance. Pretreatments prior to the oxide removal step, such as degreasing, chemical brightening, etching, and desmuting, all of which are followed by water rinsing, have essentially no influence on adhesion other than the mechanical effect caused by surface roughness. The reason therefor is that the oxide film, even if essentially removed by any of these treatments, immediately reforms upon water rinsing.
The two best electrolytes for removing oxide are those containing the OH ion and the F ion. It is thus no coincidence that historically the best pretreatments for aluminum were based on solutions containing sodium or potassium hydroxide or hydrofluoric acid. Aluminum plating processes began in 1896 and have included U.S. Pat. No. 1,627,900 (the zincate process) in 1927, the phosphoric anodizing process in 1934, the oxalic anodizing process in 1934, the double zincate process in 1939 as disclosed in U.S. Pat. No. 2,142,564, and the stannate process in 1965 which was improved upon according to the disclosure of U.S. Pat. No. 3,274,021.
All of these processes, except anodizing, rely on the removal of the oxide film by chemical dissolution and the protection of the aluminum surface from re-oxidation by a chemically deposited coating of zinc or tin, and then, for tin, by proceeding to an electrolytic strike without rinsing. The anodizing processes rely on building a relatively thick and porous aluminum-oxide coating which anchors or "keys" a subsequent electrodeposit. The non-anodizing processes attempt to achieve a metallurgical bond between the atoms of the substrate and the deposit while the anodizing process relies on mechanical bonding which requires meeting two criteria: a measurably thick oxide coating and a consistently large pore volume.
Prior art electrolytic and immersion baths for the stannate process are typically made up by dissolving stannate values (such as stannate salts, e.g., potassium stannate and sodium stannate), alkali metal hydroxides (such as potassium hydroxide or sodium hydroxide), and polyhydroxy carboxylic acid anion values (such as polyhydroxy monocarboxylic or polycarboxylic acids or salts thereof, e.g., Rochelle salts), in water so that the resulting solution contains from about 0.06 mole/l to saturation (typically about 0.25 mole/l) of the stannate salt, about 0.5 to 12 g/l (equivalent free KOH), typically about 4 g/l, of the alkali metal hydroxide, and from about 0.01 mole/liter to about 0.25 mole/liter (equivalent acid salt) of the acid anion values. The resulting tin content of these baths is about 10 g/l to 70 g/l, preferably about 30-40 g/l.
Other suitable polyhydroxy carboxylic acid anion values besides those mentioned are set forth in U.S. Pat. No. 3,274,021 to J. C. Jongkind, et al, the disclosure of which is hereby incorporated herein by reference. Versenes, including ethylene diamine tetraacetic acid, may also be added to form the basic bath, as taught in Canadian Pat. No. 811,131 to L. P. Gowman, et al, the disclosure of which is also hereby incorporated herein by reference.
The prior art baths may typically be used interchangeably for electrolytic and immersion plating. Electrolytic plating is conventionally carried out at a potential of about 2-6 volts and a current density of about 30 amps per square foot.
Deposition of tin in the stannate process releases free hydroxide. Thus the continued operation of a stannate plating bath tends to develop inreasingly thick tin deposits unless the free hydroxide is neutralized by adding an acid or by incorporating anions of polyhydroxy carboxylic acids. Typically the free KOH is at an operating level of about 5 g/l.
Many uses for plated aluminum are currently developing in the automotive industry which is undergoing rapid changes because of federally mandated requirements for increased fuel economy each model year until 1985 when the fleet average must equal 27.5 miles per gallon. Because substituting aluminum for steel saves 2.25 lbs. for every pound of aluminum used, the industry is taking every feasible means to effect a weight reduction in its entire fleet and is particularly attempting to expand its use of high strength-to-weight ratio materials, such as high-strength bright-anodized aluminum bumpers and bright chrome-plated aluminum bumpers.
The processing sequence used for electrolytic stannate pre-plate conditioning of aluminum automobile bumpers is as follows:
Buff, alkaline degrees, mild alkaline etch, desmut, tin activate, transfer without rinse, bronze-strike with live entry, water rinse, acid rinse, and plate. The transfer operation between the tin activation solution and the bronze strike solution is carried out without an intermediate rinse in order to prevent re-oxidation during the rinse. This transfer operation proceeds without difficulty when small objects are being prepared for plating because they can be quickly lifted, moved, and lowered into the strike bath. However, when large and cumbersome objects such as aluminum automobile bumpers are being handled, the mechanical operations that are required cause the transfer time to be unduly extended, thus creating a problem that has arisen specifically in the chrome plating of aluminum bumpers which are made by successively plating with tin, bronze, copper, duplex nickel and then chrome.
Stated generally, the problem arises because when an article to be plated is withdrawn from a prior art alkaline bath, tin tends to come out of solution in the bath film which remains on the part. Within about fifteen seconds from the time that the part is removed from the bath, the surface of the part beings to be coated with a loose, flaky layer of tin which renders the part unsuitable for many commercial purposes and specifically for subsequent plating operations.
With respect to aluminum automobile bumpers, the transfer time from one tank to another during automatic plating operations can take as long as 45 seconds, causing the upper areas of the plated work to develop non-adherent dark deposits of tin during transfer, resulting in blistered deposits of bronze. This problem was slightly alleviated in prior art baths where the free KOH level was noted to have been slightly raised from 5 g/l to 6 g/l, and the transfer time was extended to slightly less than 30 seconds for most aluminum alloys, including relatively pure aluminum such as 1100, magnesium-containing alloys such as 5052 and 6061, copper-bearing alloys such as 2024, and zinc-containing alloys such as 7075 and 7005. For some of these alloys, the improved prior art process can tolerate transfer times of up to 45 seconds.
However, some alloys, including the important zinc-containing alloys 7029 and 7129, continue to manifest this problem for transfer times above 15-20 seconds. 7029 alloy has the following commercial composition: 0.10% Si, 0.12% Fe, 0.03% Mn, and 0.03% Ti as maximum amounts; 0.5-0.9% Cu, 1.3-2.0% Mg, and 4.2-5.2% Zn; 0.03% maximum for each other constituent; and 0.10% maximum for a total of all other constituents. 7029 alloy is commercially available in a thickness range of 0.100-0.250 inch, average Brinnel hardness (T.sub.5) of 127 UTS (T.sub.5) of 62 ksi, and YS (T.sub.5) of 55 ksi. 7129 alloy is basically the same as 7029 alloy except that it has greater impurity level tolerances. The composition limits for 7129 alloy are: 0.15% Si, 0.30% Fe, 0.10% Mn, 0.05% Ti, 0.10% Cr, 0.03% Ga, and 0.05% Va as maximum amounts; 0.50-0.9% Cu, 1.3-2.0% Mg and 4.2-5.2% Zn; 0.03% maximum for each other constituent; and 0.10% maximum for a total of all other constituents. These alloys are presently favored alloys for manufacturing aluminum automobile bumpers.
A practical process and composition for activation treatment and tin plating of large aluminum objects, particularly when made of alloys 7029 and 7129 is accordingly needed for automatic plating operations.