GB1601057A - Plating processes - Google Patents

Description

(54) PLATING PROCESSES (71) We, FORD MOTOR COMPANY LIMITED, of Eagle Way, Brentwood, Essex CM13 3BW, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electrodeposition processes.

Considerable attention is being devoted to weight reduction in automotive design in order to conserve energy. In the case of automobile bumpers, it is necessary to reduce weight and meet governmental bumper impact standards without sacrificing durability, appearance, cost-effectiveness and styling versatility of nickel-chromium plating.

High strength aluminum alloys present one promising avenue to reducing the weight of bumper systems and at the same time meet the governmental bumper impact standards. However, is it not possible, prior to this invention, (1) to commercially electroplate a consistently adherent metal coating directly on aluminum, particularly a brass coating, and (2) to provide a bright lustrous coating system for an aluminum substrate which experiences minimal lateral corrosion. This is due to several problems among which include the natural oxide film that is present upon aluminum and the interference caused by such oxide film in achieving a sound adherency between any plated material over the aluminum base.If the natural oxide film is somehow removed and replaced by a plated system, the natural corrosion resistance of aluminum is sacrificed and the plated materials become a potential galvanic couple in a corrosive environment. In such a couple, aluminum will become the anode and will tend to dissolve. Since aluminum is more reactive than steel, the dissolving rate can actually be faster than with steel bumpers.

The performance of plated aluminum can be influenced by the pre-plating treatment or underlayment system, both referred to hereinafter as pretreatment Over the years, a number of pretreatments have been proposed, most directed to the problem of achieving high adherency. Only a few have been successful and these only to a small degree.They include (a) a chromium on nickel on copper on zinc on aluminum system (typically referred to as the zincate process), (b) a chromium on nickel on bronze on tin on aluminum system (typically referred to as the Alstan process), (c) a chromium on brass (high zinc content) on aluminum system (referred to as the Dupont process), a chromium on nickel on an immersion zinc layer (which is dissolved to some degree during immersion in the nickel bath) on aluminum system (referred to as the Alcoa 661 process), and (d) a phosphoric acid anodizing treatment wherein a chromium on nickel on anodic oxide on aluminum system is employed. Each of these systems are deficient either because they do not achieve proper adherency or they create excessive galvanic couples which accelerate corrosion between the various elements of the system.

Most of the prior art top-coat systems of decorative use here, such as nickel and chromium, were developed for use on mild steel substrates and-have found particular utility therein; the top-coat systems were subsequently transplanted for use with aluminum in the hope that their performance would be comparable. However, it must be recognized that different physical parameters do exist when a plating system is applied to aluminum. An electromotive force may exist between any of the elements of these plating systems when tied to aluminum that may not exist when tied to steel. The natural oxide coating on aluminum inhibits a tight adherency of the plated system.

Without proper adherence, the nature of the galvanic couple therebetween may be increased or decreased due to the change in the current flow between the electrolyte of the galvanic couple and the particular metal forming the poles of the couple. For example, although stainless steel is much more noble than say copper, its effectiveness in the galvanic couple is considerably greater because of its high resistance to current flow through its interface with the electrolyte. Thus, plating systems for steel substrates must be carefully analyzed because of the unpredictable nature of applying such systems to aluminum and still achieve comparable results with respect to adherency and corrosion resistance.

In a separate path of technology and in an effort to cut down on the degree of electromotive potential between the elements of the system thereby hoping to improve lateral corrosion resistance, brass has been introduced to two known instances by the prior art for use in plating steel. In the first instance, the brass was constituted to contain a high proportion of zinc, about 70%, with copper maintained at about 30%.

If this use of brass were to be applied to an aluminum substrate, zinc, being a highly reactive metal would become sacrificial and corrosion would proceed very rapidly laterally in the brass layer producing peeling and blistering under the decorative coating. In addition, the system would be limited to undesirable immersion coating techniques since consistent adherency of such a brass on aluminum by electroplating is not possible by the state of the art.

In the second instance, it was conceputalized that the electromotive potential of brass should be increased so that it is between steel and nickel; any galvanic couple created would be less and thereby slow down the rate of corrosion. This was implemented by increasing the copper content of the brass to 45-60%. If this concept were to be applied to an aluminum substrate, adherency of electroplating would still remain a severe problem and the galvanic couple between the increased-copper-brass and the aluminum would be greater than on steel; the brass would not be able to provide preferential cathodic protection to the decorative outer layers.

Accordingly, what is needed is a pretreatment which (1) provides for consistently good adherency of electroplated elements on aluminum, (2) reduces the galvanic couple potential between the aluminum and the most adjacent layer.

The present invention provides: A method of plating an aluminum or magnesium based article to provide improved adherency of the plating, comprising: (a) selecting said article to contain 18% alloyed zinc in the surface to be plated, (b) cleaning said article surface to be substantially free of any oxide film, (c) immerse said article in one or more electroplating cells with the voltage on, the electrolyte of said cells being constituted to deposit at least a brass strike directly onto the article, said strike having 2075% copper by weight Where the alloy comprises a magnesium alloy containing 18% zinc, by weight, the layer of brass deposited thereon preferably contains 40 to 60% by weight of copper.

Where an aluminum alloy containing 14% zinc, by weight is used, the brass layer preferably contains 4575% by weight of copper.

In the preferred embodiments, the invention contemplates the use of an aluminum alloy substrate containing 18% zinc, the zinc intermetallic compounds in the substrate forming, during processing, a tight intermolecular bond with selected materials of a pretreatment system, which particularly must include brass as an initial strike having a copper content of 20^75%, (b) eliminating lateral corrosion within the plated system by employing either one or all of (i) recrystallized aluminum alloy;; (ii) an aluminum substrate containing 4.0-5.5 zinc, (iii) constituting the metal strike in contact with the substrate to have 60675% copper and the remainder zinc, (c) constituting a brass pretreatment plating solution of cyanide compounds and having a high content of copper, the solution having ingredients proportioned to provide for better throwing power and tightness of the plated material, such as maintaining the ratio of sodium cyanide to metal in the range of 1.2-1.6 times the metal and avoiding an arsenic impurity above .0001% by weight, (d) insuring a controlled type of corrosion, which is substantially confined to micro-pits oriented vertical to the plated surface or controlling the corrosion to permit a slow rate of lateral corrosion in an intermediate layer by enveloping a copper pretreatment layer in two layers of brass, the brass layer most adjacent the aluminum containing 60-75% copper and the layer most adjacent the decorative plating system comprising 50-60% copper by weight, (e) the maintenance of a bright lustrous decorative finish by employing lustrous copper or copper and nickel as an undercoating for nickel and chromium overlayers, while at about the same time promoting corrosion protection by enveloping such copper or copper and nickel layer with brass containing a high content of copper, and (f) restricting cleaning and plating techniques to that which are interchangeable for plating steel.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a graphical illustration of various prior art coating systems on aluminum and the coating system of the present invention; Figure 2 is a graphical illustration similar to that of Figure 1, for prior art coating systems employed on steel or iron; Figures 3 and 4 are schematic diagrams of the sequence of corrosion as it proceeds through a coating system of the prior art commonly referred to as the zincate treatment Figures 5 and 6 illustrate graphically the progress of corrosion for another prior art coating system commonly referred to as the Alstan process; Figures 7 and 8 illustrate the progress of corrosion for a coating system in conformity with the present invention;; Figure 9 is a photograph of samples exposed to the CASS test, said samples having varying as-deposited copper contents in the brass strike applied to a 7029 aluminum alloy.

Figures 10 through 17 respectively represent photomicrographs of samples prepared according to the invention and those outside the invention, showing the presence or absence of blisters and other types of corrosion defects.

The invention is concerned with employing aluminum or magnesium as a light weight substrate upon which is plated a bright lustrous decorative metallic finish, typically comprised of nickel and chromium. Both aluminum and magnesium present, to varying degrees, the same problem of adherency of a plated system thereon due to their inherent protective qualities. Similarly, both present somewhat the same problem with respect to galvanic corrosion since they are comparable in the electromotive series and would present similar galvanic coupled with respect to the various types of plating systems that have been employed by the prior art. Hereafter, when reference is made to aluminum, magnesium shall also be considered as included unless noted.Given aluminum as the substrate, the desire for a bright lustrous decorative outer plating system, and the need for a low cost approach to the coating system, the process must provide excellent adherency and low lateral corrosion qualities.

Adherency Direct current electroplating of metals directly onto aluminum has not become commercially successful and only direct plating of chromium has become feasible.

Instead, most electroplating on aluminum in commercial practice is carried out by use of an intermediate chemical immersion layer of zinc, commonly applied by the zincate process, or by use of other immersion layers of bronze or tin. Selected immersion layers such as zinc, brass, Zn-Ni, and tin chemically displace the oxide film on aluminum which then provides a base for adhering a plating of other metals. Unfortunately, the immersion methods are more an art than a science because the actual mechanism for adhesion is not well understood and undesirable variances appear. Other metals won't even adhere by the immersion method, such as nickel, copper and iron.

The difficulty of plating on aluminum was examined extensively by Schwartz and Newkirk at the University of Colorado in 1972 and they concluded that DC plating on aluminum with the most desirable plating metals, such as brass or copper-lead, was not feasible. Svendalah of DuPont in 1974 also investigated DC plating of brass onto aluminum and concluded that the electrolyte must be adjusted to constitute the plate almost entirely of zinc before good adhesion is obtained, assuming some mode of dissolving the oxide film has occurred. Thus, the DuPont approach almost duplicates a zinc immersion method. This is further undesirable because the plated hi-zinc brass is readily attacked and dissolved in subsequent acid dips or plates necessary to plating nickel if not protected by additional barrier elements.

This invention has found that good adhesion can be reliably obtained by DC plating of brass directly onto aluminum provided the aluminum substrate is selected to contain 18% alloyed zinc and the brass electrolyte is constituted not only to deposit out brass having considerable copper 2075%, but also to contain adequate caustic elements which dissolve the oxide film and some of the aluminum. It is believed that upon entering the brass electrolyte with the current on, some alloyed zinc in surface regions of the aluminum article is redistributed to form a zinc-rich intermediate region in the plating promoting adherency of the brass. The phenomena appears to draw zinc only from the substrates to promote a more compatible crystal structure uniting the substrate and plate.An absence of alloyed zinc in the article along with some or increasing zinc in the brass electrolyte did not provide for improved adherency.

In addition, the high copper brass is not attacked by subsequent acid dips or plates necessary to plating nickel and thus no intermediate barriers need be used.

A preferred method of carrying out this invention is as follows: 1. Provide a wrought or extruded aluminum article or substrate having 1-8% alloyed zinc; lesser amounts of alloyed zinc affects adhesion and greater amounts of zinc undesirably affect the physical characteristics of the aluminum.

2. Subject the aluminum article to the following cleaning and activating cycle which removes foreign matter and oxide films from the article: (a) soak in a mild alkaline cleaning solution for 1-4 minutes at 140-1800F, and power spray with a similar alkaline solution at 110-1300F; (b) etch the article in another alkaline water solution preferably containing 68% sodium hydroxide, 0.5% min. trisodium phosphate, 15% sodium metaphos phate, and 10% max. sodium carbonate; (c) de-smut the article in a sulfuric acid solution (212% by volume) with added fluoride salts and/or hydrogen peroxide; water rinsing at room temperature is applied to the article after each of the soak, etch and de-smut steps.

3. Immediately subject the cleaned and etched article to a DC plating cell with the article arranged as the cathode, to plate on said article a thin brass strike (.00005-.0001 inch) which contains 20-75% by weight copper as deposited.

The electrolyte is a cyanide solution and carries the full plating voltage at the instant the article enters; plating should be carried out for 3-10 minutes at preferably 30-50 (or operably 20-60) amps./ft2 as the average current density. The electro lyte should preferably contain: 2-5 oz./gal. sodium hydroxide 4-12 oz./gal. sodium carbonate 3.6-9.6 oz./gal. free sodium cyanide 3-6.0 oz./gal. metal elements divided between copper and zinc in the proportion of 68-85 copper and 1532% zinc, the ratio between the sodium cyanide and metal being 1.2-1.6 times the metal.

Several tests examples will bear out the importance of the alloyed zinc in the alu minum substrate and the copper content of the brass for achieving adhesion. The sample preparation consisted of selecting various lots of aluminum alloy panels (about 4"X4"), including heat-treatable and non-heat treatable alloys, and; (a) buffing one side of each panel (in the case of clad materials, the bottom half was buffed only); (b) applying acetone to remove buffing compound; (c) soaking and brushing clean each panel in a solution at 1600F prepared with 8 oz./gal. of a powdered additive consisting by weight of 2.5-3.5% sodium metasilicate, 17.5-18.5% sodium pyrophosphate, 31.0-32% sodium tetra borate, 0.5-1.5% surfactants and wetters, and 23-24% complex organics; (d) rinse; ; (e) soak in a solution at 160 F for 1/2 minute, the solution being prepared by use of 8 oz./gal. of a powdered additive consisting of 68% sodium oxide, 15-20% sodium metasilicate, 0.5% max. trisodium phosphate, 10% max. sodium, carbonate, balance sodium hydroxide, and 3.5% max. moisture; (f) rinse; (g) soak again in a solution as in (e) at 160"F for 1/2 minute; (h) rinse; (i) dip in a water solution of 50% HNO, for 15 seconds; (i) rinse; (k) placing in electrolyte of brass plating cell with current on, using the panel as a cathode; average current density is about 35 ASF and the electrolyte con sisting of (all in oz./gal.) 5.8 free NaCN, 2.0 NaOH, 8.0 Na2 CO2, 2.9 Cu, 1.5 Zn. Plating is carried out for 4 minutes. The as-deposited copper content of the brass was about 57%.For certain samples, as noted, the brass copper content was varied; (1) rinsing (if adherent, the plating is carried out for another 9 minutes; visual check only); (m) placing each panel in an acid copper plating cell with current on for 5 minutes' to plate a copper strike thereover; (n) rinsing; (o) determine adhesion qualitatively by 1 - rapping with coarse file, 2 - sawing 3 - bending panel by folding back upon itself (180 bend).

The results of the tests, as shown in Table I, clearly show that with aluminium substrates containing 18% zinc (see Table II), adhesion became excellent, regardless of the other alloying elements.

TABLE I Visual Inspection of Check of Adhesion Adhesion after Brass after Copper Plate Panel Alloy Strike by 3 Tests 2024 Looked OK Blisters and peeling 2036 No adhesion No adhesion 3003 No adhesion No adhesion 3105 Blistered and peeled Blisters and peeling to Al.

4343 Blistered and peeled Blisters and peeling peeled to Al.

5457 Peeled to Al. Peeling 5557 Peeled to Al. Peeling 6061 Very heavy blistering Poor adhesion Reflectal R-5 Peeled to Al. Peeling Ford Casting Alloy 332 Peeled to Al. Peeling Ford Casting Alloy 103 Peeled to Al. Peeling 7046 Looked excellent Excellent 707 5/707 2 Clad Looked excellent Excellent 4343/7072 Clad Looked OK on 4343 Peeled to Al. on 4343 and looked OK on 7072 check was excellent on 7072 7178 Appeared excellent Excellent 7075 Appeared good Excellent 7016 Looked excellent Excellent 7029 Looked excellent Excellent TABLE II Nominal Chemical Analysis - % by weight, other than Al

Al. Alloy Si Cu Mn Mg Cr Ni Zn Ti Zr 2024 - 4.4 - 1.5 - - - - 2036 - 2.6 .25 .45 - - - - 3003 - .1 1.2 - - - - - 3105 - - .6 .5 - - - - 4343 7.5 - - - - - - - - 5457 - - .3 1.0 - - - - - 5557 .1 .15 .25 .6 - - - - - 6061 .6 .3 .15 1.0 - - .25 - Reflectal R-5 - - - .5 - - - - - Casting Alloy Ford 332 Alloy 9.5 3.0 .5 1.0 - - - - - Ford 103 Alloy 9.5 3.0 .5 1.0 - - - - - 7046 .1 max 1.0 - 1.1 - - 4.5 .06 max .1 7075 - 1.6 - 2.5 .26 - 5.6 - 7072 - - - - - - 1.0 - 7178 .5 2.0 .3 2.7 .3 - 6.8 .2 - 7016 .1 max 1.0 - 1.1 - - 4.5 .03 7029 .1 max .7 - 1.6 - - 4.7 .03 max - Lateral Corrosion Figure 1 graphically summarizes pertinent coating systems for aluminum that have been used by the prior art; in comparison, two inventive modes are also illustrated.Three of the prior art systems (Z-1, A-i and AI-2) use immersion or chemical conversion coatings to obtain adherency to aluminum. Z-1 is a commercial zincate process which is referred to in some detail below; A-i is a proprietary system employing a stannous bath that creates difficult process control. The Al-2 system is sometimes known as the Alcoa 661 process and presents difficult problems of dissolution of the zinc coating in the nickel treating bath. The ON-i or anodic oxide process does not give good tight adherency of the coating system.The Du-1 (DuPont) process uses an electro-plated white brass layer that contains about 90% zinc; such brass layer encourages significant lateral corrosion since elemental zinc is quite sacrificial.

In Figure 2, pertinent prior art plating pretreatments for use on steel is summarized. There is little problem to obtaining good adhesion on steel and direct DC plating from cyanide solutions is commercial. Copper or nickel have been used as the interface layer with steel, but copper and nickel do not adhere to plate on aluminum.

Thus the plating technology used on steel is not transplantable for use on aluminum.

The brass intermediate layer in prior art system P---l, even if containing 6075% copper, would not have the same corrosion protection that exists when the brass is directly coupled to the steel.

It should be noted that aluminum, by itself, is quite corrosion resistant because of the protection resulting from its natural oxide film. After being plated, aluminum no longer enjoys this natural protection. Plated aluminum is part of a Potential galvanic couple. Aluminum will be the anode in most galvanic couples and will tend to dissolve (except with zinc rich layers where aluminum is cathodic). Since aluminum is more reactive than steel, the dissolving rate can actually be faster than with steel.

The other metal or metals, will be less reactive and will tend to plate hydrogen gas; this is the electrolyte solution which most likely consists of water with ordinary road salt and sulfuric acid as the conductive materials. The relative speed of corrosion is in part related to the voltage of the galvanic couple. The voltage in turn depends on the reactivity of the metals involved.

One prior art process that illustrates the galvanic corrosion problem is that of the zincate process, wherein after suitable cleansing of the aluminum substrate, an immersion layer of zinc is applied typically in the thickness range of .000001-.000005" (See Figure 1). After a rinse, copper strike is electro-deposited thereover followed by the decorative coating of nickel and chromium, the nickel being usually about .001". The zinc is the most troublesome and vulnerable layer. Zinc is readily attacked and dissolved in the acid processing steps where nickel or acid copper plates are applied; accordingly some barrier deposit (such as thick copper) is necessary which adds cost The zinc anode is electrically connected to a very efficient large area cathode made of copper.As soon as the corrosive solution reaches the zinc layer through slight cracking (See Figure 3) the latter will dissolve preferentially. This occurs very readily with just a slight scratch or pit in the plated coating. The corrosion rate will increase gradually. The area of the zinc anode will be merely the thin edge of zinc layer. The copper cathode is much larger. As the zinc dissolves away from the pit, it will produce under-cutting and thereby the anode area exposed will increase as the circumference of a circle increases. There will be an accelerating effect. The anode cathode area ratio is steadily declining which means that the corrosion rate progressively rises. Once the zinc is substantially diminished or taken from the area, corrosion does not stop. The copper-aluminum couple is noted for corroding aluminum.Substantial dissolving of the aluminum occurs as long as the fresh electrolyte is available. The depletion of the electrolyte is about the only retarding factor in this galvanic couple. If the plate is ductile and does not rupture from the build up of the corrosion products, there may possibly be an eventual slow down of the corrosion. But usually the corrosion products proceed to such a catastrophic dimension that peeling and blistering of the plated system occurs. As a result, there will be wide spread lifting of the plated material (see Figure 4), unsightly voluminous corrosion products beneath the plate and exuding of corrosion products from the ruptures, and the eventual weakening of the aluminum base.

Turning now to Figures 5 and 6, there is illustrated another prior art plating system which has been applied to aluminum, although it has achieved some success with respect to plating steel. This is a system relying upon a thin immersion layer of tin and a layer of bronze (.0000I which in turn is followed by a nickel chromium plating. When corrosion does occur through a slight crack, scratch or pit, the corrosion will proceed vertically (that is perpendicular to the aluminum surface) and when it reaches the aluminum through a break in such coating, a galvanic couple immediately is operative between the aluminum and bronze. The area of the cathode initially is confined to the periphery of the pit. This is true especially of chromium as the top layer. Corrosion can continue almost indefinitely.However, the anode/cathode area ratio is larger. Also the potential gradient in the galvanic battery is smaller because aluminum and bronze are quite close together in the reactivity scale, and thus the corrosion rate is somewhat lower. However, corrosion will proceed by dissolving of the aluminum which will produce an undercut area beneath the bronze. The rate of attack around the periphery of the pit measured in depth of metal consumed for unit time will tend to decline. The cathode at the offset will consist mostly of the nickel with but a sliver of bronze exposed edgewise. As the corrosion progresses, and undercutting develops successfully, more and more bronze plated nickel is exposed, the cathode gradually should become more efficient and the corrosion should increase. This system of course allows for the substrate to be directly attacked and left somewhat unprotected.

In the case of an anodized process for plating aluminum, nickel will remain the cathode, irrespective of how far the corrosion has progressed. The corrosion rate will be somewhat reduced. However, the underside of the nickel is attached to the bottom of the aluminum through the pores of the oxide. The disadvantage of the process is that the plating adhesion is poor. Corrosion products build up and pry the plate loose and when this happens, more cathode is exposed and corrosion speeds up.

From the above prior art, it can be appreciated that desirable or tolerable corrosion is that which (a) proceeds vertically through a plated system confining itself to a small pit, (b) never proceeds laterally through a layer of the system most adjacent to the aluminum substrate, and (c) reduces any eventual galvanic couple with aluminum to a minimum for retarding decay of the supporting substrate. The zincate process fails because corrosion proceeds laterally along the layer most adjacent the aluminum; the tin-bronze pretreatment fails because it sets up too high a galvanic couple with the aluminum causing considerable eventual decay. The anodic oxide process fails because of a lack of tight adherency of the pretreatment with the aluminum.But more importantly the corrosion mechanism should be controlled by an economical convenient electronlated pretreatment, which is contrary to these three prior art modes.

This invention overcomes the above prior art deficiencies by (a) controlling the alloy content of the aluminum substrate to contain 46% zinc or a crystallized aluminum substrate with 18% zinc, and (b) electroplating a thin brass strike directly onto the aluminum article, the strike containing 60-75% copper to eliminate lateral corrosion along the article interface and to promote tight adherency, and to accommodate acid processing steps such as the plating of nickel directly thereonto. The plated system, in its broadest sense, is shown in Figure 1, labelled invention Mode A. The thickness of the electrodeposited layers should be about: brass .0001"; nickel .001", and chromium .000005". The reason for the high resistance of this system to lateral corrosion of the brass is not fully understood.Nonetheless, it is believed that the atomic bond between the aluminum and brass is such that vertical corrosion is mandated; there may be a zinc rich intermetallic region in the substrate between the brass and aluminum which promotes this bond and of course would change the galvanic couples of the elements of the system. The aluminum is attacked at a very low rate, and appearance is not effected because the small pits release white corrosion products and avoid peeling or blistering.

An alternative inventive plated system is shown in Figure 1, (as Mode B) and Figures 7-8. A copper or nickel and copper layer 10 (.0005") is enveloped between two brass layers 11-12, the first (11) having 6075% copper to eliminate lateral corrosion at the interface with the aluminum substrate, but the other (12) having 4560% Cu as-deposited to permit a slow, controlled sacrificial corrosion to protect the aluminum longer.

The copper 10 is pure copper and the brass layers are electrodeposited from brass cyanide solutions. The electromotive differential potential between nickel and the brass layer 12 is relatively small and the polarizing characteristic of brass (12) will be such that it does not substantially shift the small voltage potential and permits current to flow readily. Accordingly, brass (12) corrosion will undercut the nickel. This will take considerably longer than that experienced by any of the prior art methods and during this period of time, both the copper and brass (11) will be protecting the aluminum. If and when the brass (12) has been corroded so substaptially penetration will occur through the copper or copper and nickel layer (10).If the corrosion should proceed through second brass layer 11, the galvanic couple between the high copper brass (11) and aluminum will be relatively small encouraging corrosion to proceed at a slower rate, if at all. The layer 11 will experience at most very slight sacrificial corrosion (lateral corrosion) to the Cu above; this is important to controlling corrosion to that which is vertical and least objectionable to appearance. Corrosion will proceed through the copper and brass in a substantially vertical direction, limiting the products of corrosion, and preventing peeling or breaking away of the plated layers due to blistering and lack of adherency.

A preferred detailed process Mode B would be as follows: 1. Select and employ a recrystallized aluminum substrate which is of the 7000 series containing 46% zinc.

2. Soak the aluminum alloy to provide a rough general surface cleaning. This soaking treatment may be carried out in three phases, (a) soaking in a mildly alkaline cleaning solution as in step (c) for the test examples, for a period of time of 1-4 minutes at a temperature of 140-1800F, (b) power spraying the aluminum substrate with a similar mildly alkaline cleaning solution as in (a), for a period of time of 1-3 minutes at a temperature of 110--130"F, the power spray being carried out to direct the solution against the aluminum substrate with a force of about 16 psig, and (c) rinsing the soaked and sprayed substrate with water for a period of one minute at room temperature.

3. Subject the soaked aluminum substrate to a mild etching cleaner for producing an even etching of the aluminum surface. The etching solution is an alkaline, mildly or non-silicated, electrocleaner or similarly formulated alkaline solution that provides an even etch on the surface when the aluminum is subjected for a period of time of 1-3 minutes; the solution being maintained at a temperature of about 100-1500F.

A preferential solution preparation may comprise; adding a powder in the proportion of 6-11 oz./gal. of water, the powder additive containing a maximum of 3-5% moisture, the powder including 68% sodium hydroxide, 0.5% minimum trisodium phosphate, 15% sodium metaphosphate, and 10% maximum sodium carbonate. The aluminum is then subjected to a water rinse to remove the products of the mildly etching alkaline solution, the water rinse being carried out for about 2 minutes at room temperature.

4. De-smut the etched and soaked aluminum alloy by immersion or dipping in a mild acid solution for a period of about 1 minute, the solution being maintained in the range of 60-80 F. A preferential de-smutting solution may contain 2-12% by volume of sulfuric acid with added fluoride salts, such as 0.25 oz./gal. ammonium bi-fluoride, and/or hydrogen peroxide. Rinse away the products of such de-smutting treatment by immersion in water for a period of 1 minute at room temperature.

5. Electrodeposit a brass strike onto the prepared aluminum substrate; immerse the article in the electrolyte with the full plating voltage on, the average plating current density being about 30-50 amps./sq./ft. Electrodeposit for 3-10 minutes with the electrolyte at a temperature of 70-900F, to obtain a strike about .0001". It is important that the brass strike deposit contain a high copper content specifically in the range of 6v75% copper and the remainder zinc. To this end, the electrolyte must be comprised of: (a) sodium cyanide 4.6-7.5 oz./gal.

(b) sodium hydroxide 2-4.0 oz./gal.

(c) sodium carbonate 4-12 oz./gal.

(d) total metal in solution including both copper and zinc being 3-6 oz./gal.; the copper, as a percent of the total, being 6885% in solution and zinc, percent of total, being 3215%. This may be achieved by employing zinc cyanide at about 5.0 oz./gal. and copper cyanide at about 5.0 oz./gal. There is no need to include a brightener in the brass plating electrolyte such as di methylsulfamate or sodium polysulfide. Rinse the electroplated substance in water at room temperature for about 1 minute.

6. Electrodeposit a copper or copper and nickel layer about .0005". The copper layer may be deposited progressively in layers such as first (a) employing a copper strike of .00005" utilizing an electrolyte having a general composition of 5.3 oz./gal.

CuCN 6.7 oz./gal. NaCN, 4 oz./gal. Na2CO3 and 8 oz./gal. KNaH4O6 . 4H2O; (b) plating an acid copper layer from a copper sulphate and sulfuric acid electrolyte, the thickness being about .0004", and (c) plating a cyanide copper strike to a thickness of about .00005" (rinsing being provided after each of the copper layers).

7. Electrodeposit a high copper brass layer, thickness about .0003", containing copper in the range of 5060%. The coated substrate from the previous steps is placed in the electrolyte without the current on, the current density being about 30-60 amps./sq./ft. and plating is carried out for a period of time of about 30 minutes to provide said thickness. The electrolyte preferably contains sodium hydroxide of 3.5 oz./ gal., free sodium cyanide 6.5 oz./gal., copper cyanide 4 oz./gal., zinc cyanide 2.5 oz./gal. The substrate from this step is also rinsed with water at room temperature for about 1 minute.

8. Electrodeposit a copper layer thickness of about .00005" from a cyanide copper strike to insure optimum adhesion between the 5060% copper brass layer and the following nickel layer. Rinse in water.

9. The substrate from the previous steps is then dipped in an acid containing 1% H2SO4 (by vol.) for a period of time of about 1 minute.

10. The previously plated substrate is then provided with an electrodeposit nickel plating to a thickness minimum of .0003", the nickel being bright and the nickel electrolyte being preferably comprised of 40 oz./gal. NiSO4. 6H20, 18 oz./gal.

NiCl2 6H2O, 6i oz./gal. H,BO, with brightening and wetting agents: the nickel plated substrate is then rinsed in water.

11. Finally the substrate is provided with outer chromium plating to a thickness of about .000005" in an electrolyte containing preferably 45 oz./gal. CrO3 and .4 oz./bal. H2SO4, and employing a current density of about 175 A.S.F. The chromium plated substrate is then hot rinsed in water at about 19e-2000F and dried by blowing hot air thereover.

A series of test examples demonstrate the improved resistance to lateral corrosion of the inventive modes; the results of the tests are tabulated in Tables III and IV.

The examples in Table III had the as-deposited copper varied in the first brass layer, but the substrate was consistently 7029 aluminum alloy. In Table IV, the substrate and mode was varied, along with % as-deposited copper. It is evident from these tables that to obtain no lateral corrosion after 64 hrs. of the CASS test, the substrate must contain 46% zinc, and the first brass layer must contain 6075% Cu as deposited.

It is further evident, as shown in Figure 9, that only that sample containing 63% copper had a sharply defined scribe mark after 64 hrs. CASS test.

For these samples, the corrosion performance of plated aluminum alloy panels or sections were studied by use of the CASS test using the standard procedure specified in ASTM B-368. This test essentially comprises exposing the cleaned plated panels for increasing periods of time to a fog in a closed chamber. The fog is generated from a salt-copper solution adjusted to a required PH with glacial acetic acid. The test cycles were for 16 hours each and four such cycles were used. Prior to testing, each panel or section was scribed with an x-pattern; scribing was by a carbide cutting tool which cut through the plating into the base metal. The panels or sections were then sectioned through the scribe and photomicrographs taken of the corrosion progress, if any.

Physical Characteristics of the As-Plated Product Figures 10-17 are photomicrographs of the sectioned scribe marks in 7029 aluminum panels B-S, G-1, G-3 and G-7 of Table III. Figure 16 (at 50 x ) shows that 26% Cu in the brass causes considerable lateral corrosion and delamination at 13, the tip of said lateral corrosion is enlarged in Figure 17. In Figure 14 (at 50 x ) the sample contained 42% Cu in the 1st brass layer and this permitted again considerable lateral corrosion, the magnification of the corrosion progression is shown in Figure 15.

In Figure 12, the use of 56% Cu in the brass strike still allowed some lateral corrosion to proceed from the scribe along the brass. Only in Figure 10 and 11 (for 63% Cu) do we see a tight adherency between the brass strike 14 and aluminum 15 with no lateral corrosion.

TABLE III

Test Sample CASS RESULTS Brass Strike Copper & Copper Strike 16 Hours 32 Hours 48 Hours 64 Hours Corrosion Corrosion Corrosion Corrosion At At Blister At At Blister At At Blister At At Blister Bash Deposit Scribe Edge Diameter Scribe Edge Diameter Scribe Edge Diameter Scribe Edge Diameter G-1 58 55.9 No 1MM - 2MM 4MM 1MM - 4MM 3MM - 4MM 4MM 4-5MM G-2 53 52 No 2MM - - 2MM 1MM - 2MM 1MM - 3MM 2MM G-3 45 41.8 Yes None None None 2MM 2MM - 2MM 3MM - 2MM 5MM G-6 29.5 32.0 Yes < 1MM - 10 1MM 1MM- 3MM 2MM - 3MM 5MM 15 large blister blisters each and 3 1MM small blisters < 1MM-4MM G-7 20.3 26.0 Yes None None Many 2MM 2MM - 3MM 4MM - 4MM 5MM Many small < 1MM-4MM blisters < 1MM B-8 75 63.0 Yes None None None None None None None None None None None None C-8 75 63.0 No None None None None None None None None None 1MM None None TABLE IV

% Cu in Electroplated Brass Strike CASS Results Sample Alloy Pretreatment Top Coating System (1st Layer) (visual inspection from top) RD 18 7016 Alstan tin immersion 52% Brass, Ni and Cr OK OK OK 1/32" RI 18-3 7106 " " " " " 1/16" 1/8" 1/4" 1/4" RM 184 7016 " " " " " 1/16" 1/8" 3/16" 1/4" 7029 " " " " " OK OK OK OK Electroplated Brass Strike RV 18 7046 & Cyanide Cu Strike & " " 1/16" 1/8" 1/8" 3/8" Acid Cu 21 7029 " " " " " 65 OK OK OK OK 19 7029 " " " " " 58.3 OK OK OK < 1/32" 13 7029 " " " " " 65.8 OK OK OK OK 7 7029 " " " " " 56.3 OK OK < 1/32 < 1/32 11 7029 " " " " " 58.7 OK OK OK < 1/32 6 7029 " " " " " 56.3 OK OK < 1/32 1/32 15 7029 " " " " " 65.8 OK OK OK OK 24 7029 " " " " " 65 OK OK OK OK 9 7029 " " " " " 58.7 OK OK OK < 1/32 2 7046 " " " " " 51 < 1/32" 1/32 1/16 1/16" 18 7046 " " " " " 58.3 1/32 1/32 1/32 1/16 RZ 18-1 7046 " " " " " 58.0 1/32 1/16 1/8 3/16 RZ 18-2 7046 " " " " " 58.0 1/32 1/16 3/32 1/8

Claims (27)

WHAT WE CLAIM IS:

1. A plating process comprising the steps of cleaning substantially all the oxide film from the surface of an aluminium or magnesium alloy containing from 1 to 8% by weight of zinc, and electrodepositing onto the clean surface a layer of brass containing from 20 to 75% by weight of copper.

2. A process according to claim 1 wherein the brass is electrodeposited on the said surface from a caustic solution containing cyanide ions and copper and zinc ions in sufficient proportion to plate out a deposit of brass having 2075% by weight of copper.

3. A process according to claims 2 wherein the electrolyte contains from 2 to 5 oz./gal. of sodium hydroxide, from 4 to 12 oz./gal. of sodium carbonate, from 3.6 to 9.6 oz./gal. of free sodium cyanide, and from 3 to 6 oz./gal. metal ions, said metal ions being comprised of: 6885% copper and the remainder zinc, the ratio between the sodium cyanide and metal being 1.2-1.6 times the metal.

4. A process according to any one of claims 1 to 3 wherein the electrodeposition is effected in a cell having an average current density of from 20-60 amps./ft.2, and an electrolyte having a temperature of 70-90 F and an arsenic impurity no greater than .001%.

5. A process according to any one of claims 1 to 4 wherein the said surface is subjected to etching after the removal of the oxide film therefrom.

6. A method according to claim 5 wherein the said surface is etched by contacting with an aqueous solution containing 68% sodium hydroxide, at least 0.5% trisodium phosphate, 15% sodium metaphosphate, and no more than 10% sodium carbonate.

7. A process according to any one of claims 1 to 5 wherein the oxide film is removed by (i) soaking the aluminium article in a mildly alkaline water solution for 1-4 minutes at 140-1800F, (ii) power spraying the soaked article with a similar mildly alkaline water solution at 110130 F for 1-3 minutes and (iii) rinsing said article for one minute with water at room temperature.

8. A process according to any one of claims 1 to 7 wherein the said surface is de-smutted immediately before the electrodeposition.

9. A process according to claim 8 wherein the said surface is desmutted by contact with a mild acid etching solution at room temperature for 1-3 minutes, the mild acid etching solution containing 212% by volume of sulphuric acid with 0.25 oz./gal.

ammonium bifluoride.

10. A process according to any one of claims 1 to 9 wherein the alloy comprises a recrystallised aluminium alloy containing from 1 to 8% zinc, and the layer of brass electrodeposited onto the surface contains 60 to 75% copper by weight.

11. A process according to any one of claims 1 to 9 wherein the alloy comprises.

a magnesium alloy containing from 1 to 8% by weight of zinc and the layer of brass electrodeposited onto the surface contains 40 to 60% by weight of copper.

12. A process according to any one of claims 1 to 9 wherein the alloy comprises an aluminium alloy containing from 1 to 4% by weight of zinc, and the layer of brass electrodeposited onto the surface contains from 45 to 75% by weight of copper.

13. A process according to any one of claims 1 to 12 wherein the first-mentioned brass layer is at least 0.00005 inches in thickness.

14. A process according to any one of claims 1 to 13 wherein the first-mentioned brass layer is from 0.00005 to 0.0001 inches in thickness.

15. A process according to any one of claims 1 to 14 wherein an outer layer comprising nickel and/or chromium is deposited on the surface over the brass layer.

16. A process according to any one of claims 1 to 15 wherein a second layer comprising copper or copper and nickel is deposited on the brass layer, and a further layer of brass is deposited on the said second layer.

17. A process according to claim 16 wherein the second layer has a thickness of about 0.0005 inches and the further layer of brass contains 5060% copper by weight

18. A plating process according to claim 1 substantially as described herein with reference to the drawings and/or examples.

19. An article of manufacture having a surface plated by a process according to any one of claims 1 to 18.

20. An article of manufactl.lre composed of an aluminium or magnesium alloy containing from 1 to 8% by weight of zinc having a surface plated with a layer of brass containing 20 to 75% by weight of copper.

21. An article according to claim 20 wherein the alloy is composed of aluminium and from 4 to 8% by weight of zinc, and the layer of brass contains from 60 to 75% by weight of copper.

22. An article according to claim 21 wherein the aluminium is recrystallised aluminium.

23. An article according to any one of claims 20 to 22 wherein the plated surface further comprises a coating of copper on the brass layer.

24. An article according to claim 23 wherein the coating of copper is overcoated by a decorative layer of chromium and nickel.

25. An article according to claim 23 or 24 wherein the plated surface further comprises a coating of brass on the copper layer.

26. An article according to claim 25 wherein the brass contains from 50 to 60% by weight of copper.

27. An article of manufacture according to claim 19 substantially as described in the examples or with reference to the drawings.