Improving abrasion and corrosion resistance of black finishes in the decorative sector

Introduction

Black decorative surface finishes are existing in several variations of accessible technologies and used every day covering a wide range of application methods. Though these methods have been used on an industrial scale for years, each method carries its own limitation. As the base substrates treated have a high probability to be white in color in the case of aluminum, steel, and silver, or in other cases pre-plated with a white layer used to protect the base substrate as in the case of brass and zinc alloy, abrasion resistance is a key qualitative factor used to benchmark all categories of black decorative surface treatments. As the black layer wears, the white layer is exposed, leaving a color contrast that is well defined by the human eye making it a rather simple task for the final buyer to evaluate the quality of the product as it ages. 

Specific to electroplating, black plating process have seen several modifications in recent years. With the European REACH compliance in full effect, some chemicals previously used as blackening additives or oxidizing agents can no longer be used. Metals historically used for black electroplating such as nickel, cobalt, and chromium remain under constant scrutiny, increasing awareness of brands, buyers, and government agencies resulting in requests for limited use or no use of these metals increasing year by year fundamentally in the cases where the final object is to be worn or in contact with human skin.

Some commonly used REACH compliant black processes in the decorative electroplating sector are black ruthenium and black rhodium. These treatments are generally considered flash with a maximum obtainable thickness of 0.2 micron and rather easy to be worn in time even with the elevated hardness of both metals.

This paper will follow the development of a black gold electroplating process, run comparative trials against commonly used black decorative applications in our industries today, and explore its possible advantages both as a final and technical layer,    

Black decorative finishing technologies

Numerous types of black finishing methods are available and vary depending on the original substrate to be treated and the industry where applied. Though these processes are quite abundant in number, application, design, and cost limitations come to fruition when examined.

 Anodizing is a process for finishing aluminum alloys that employs electrolytic oxidation of the aluminum surface to produce a protective oxide coating. These coatings can be colored to a limited range including black. Though anodizing has a vast functional usage industrially, the process is used heavily as a decorative method in many sectors to include home appliance, electronics, and most notably the mobile phone sector due to its cost/quality ratio. As the process is essentially a coloration of the metals natural oxide layer, anodizing is part of the aluminum itself providing total bonding and excellent adhesion properties. The principal drawbacks to the process in general is that it tends to disrupt detail therefore it use is partial to simple shapes and the application itself is limited to aluminum and very few other non-ferrous metals restraining the processes overall usage decoratively speaking.  

Paint and coatings are solvent or water diluted resin processes that provide the widest range of colors and effects of all the methods discussed. Though processing sequences may differ, paint and coating can be applied to any metallic or even plastic substrate. Due to this fact its usage can be found in just about every sector examined however the spectacle frame market can be identified as having the most mature use of paint in the higher quality decorative ornament market. Many variations of black are obtainable with this type of application.  Single layer cataphoretic coatings (e-coatings) are quite cost effective however suffer in either durability or oxidation resistance depending on the type of resin used. This set back can be circumvented with the use of 2 and 3 paint layer systems applied by spray using multiple resins, however in the end produces a higher cost due application time and a raw material loss of up to 70%. The downside to the use of paint would be the fact that it disrupts the mechanical movement of functional parts such as claps, chains, and snap hooks. In the specific case of the spray application, it bonds to any substrate it contacts therefore jewelry use is limited due to the constant presence of gemstones. Paint has also been stigmatized as “cheap” due to the plastic feel it tends to give to metal therefore many luxury goods sectors, depending on brand position, will not consider its use.

Physical vapor deposition (PVD) is a process that uses a vacuum chamber to pressurize a target into vapor which is then altered into a condensed film on the objects surface. Applicable substrates are limited to the temperature required to make the particular deposition which can be quite high limiting use. This process is able to yield several colors to include black. One of the more recent process used is diamond like carbon (DLC) which produces a jet-black layer that is extremely hard and resistant to abrasion. Initial equipment investment is rather high making PVD by far the most expensive technology discussed being difficult to find economically outside of China. Though very resistant to abrasion and wear, the chemical resistance of PVD tends to be rather weak due to the formation of microscopic pin holes formed in processing. This is eventually overcome with multiple treatments, however the cost of the treatment is also dramatically affected. Given the watch sector uses stainless steel as a base material, maintains an elevated abrasion resistance requirement, and the geometries treated are standardized, makes it the perfect candidate for PVD and it use is widespread throughout the sector.

Black electroplating treatments are vast using a large variety of metals to obtain the targeted result. Two of the most demanding decorative markets for this type of plating are the jewelry industry and the fashion accessories sector. Both sectors can be divided into high and low end segments. The low end segments utilize metals such as chrome, nickel, tin, cobalt, or an alloy thereof principally due to cost. Though these metals are cost effective, the finishing obtained is not enough to pass normative climatic testing required by some brands. This fact paired with the increasing request for nickel free finishes and stricter regulation as in the case of the European Unions REACH legislation, the higher end segments lean on metals such as palladium, rhodium, and ruthenium to obtain a black color. High end jewelry tends to use black rhodium where as high end fashion accessories utilize black ruthenium. Where black ruthenium and black rhodium are able to pass most climatic testing if used with the proper pre-plating sequences, both have a maximum obtainable thickness of 0.2-0.3 micron so tend to present abrasion resistance issues. This is circumvented at the moment in both high end sectors buy covering the metal with a paint to improve on abrasion or just by accepting the fact that the color will wear in time.

Which leads one to ask, is an abrasion resistant black layer obtainable without the use of paint, while maintaining both nickel free processing and REACH compliance? If so, can the market cost be bared?          

Nickel free black gold

Both rhodium and ruthenium have a maximum obtainable thickness of 0.2-0.3 micron mainly due to the fact that they are mono-metal systems that become brittle with thickness. With a target to improve abrasion resistance, the first objectives would be to maintain a good hardness while making the layer more malleable and opening the range of reachable thickness. Omitting the use of nickel and cobalt to remain hypoallergenic as well as the use of copper which remains a principal source of oxidation, a bi-metal electrolyte was studied that included gold, palladium, and iron. The system was studied at an alkaline pH to well receive the selected metals with gold in the highest concentration and iron with the lowest concentration of the metals used to develop the chemical.

Table 1 – Black gold electrolytic characteristics

Surface evaluation of the black gold alloy

When processed, the electrolyte produces an  alloy that is black in color and consists of 49% palladium, 39% gold, and 12% iron by weight making the deposit roughly 12.5 KT gold by title. To evaluate the alloy color, the CIELab color coordinate system is used with principal focus on the L coordinate when evaluating black finishes.

Table 2 – Color coordinate comparison of black gold, black ruthenium, and black rhodium

The L coordinate is the luminosity value which in this case determines the overall darkness of the black deposit with the lower L value equating to a darker shade of black. The L value of 58 was measured with the black gold alloy.

This fits into the higher range of the industries standard black colors with L coordinates ranging from 50-60 in the case of black rhodium and a much wider availability of black shades with ruthenium having L coordinates ranging from 32-60. The most commonly used formulas of both rhodium and ruthenium have L coordinates of 58 or 59.

Hull cell testing was conducted to identify the current density range, resulting in a mirror finished black panel produced at 1.0 A/dm 2 after 10 minutes of deposition time. This paired with the fact the electrolyte demonstrated good reach and throwing power, 1.0 A/dm 2 optimum current density value.

Thickness tests were done with the use of SEM/EDX microscope. Lab trials concluded that a level deposit of up to 2 micron was obtainable while maintaining a mirror like finish expected in the high end decorative plating sector.  With accurate thickness measurements, plating speeds were calculated obtaining 1 micron in 12-15 minutes at 1.0 A/dm².

Chemical observations

The chemical has proven stable over a 12 month duration of time exhibiting no participation chemical elements if the proper pH is maintained. A fluctuating pH was detected in the electrolyte observing the pH dropping by roughly 0.2 each day.

The lowering pH can be corrected with the addition of pH adjustment salts, however would entail that operators of the black gold solution would have to monitor the pH solution of the daily, giving the electrolyte similar maintenance characteristics to a gold sulfite bath.

Figure 1 – pH variance in an 18-day period

Market segment plating sequences

Two of the more mature and standard black plating sequences were evaluated in each high-end market segment. In the case of jewelry, black finishing over silver was assessed as one of the more standardized cases. Starting from a base material of .925 silver, palladium is applied as the initial plating layer followed by black rhodium in the most common finishing cycles. In this example, palladium is applied as a technical layer serving a dual use. The first improving the overall resistance to oxidation acting as a barrier to copper migration as well as preventing corrosion from environmental factors. The thicker the palladium layer, the stronger is the resistance to climatic testing. The second reason for palladiums use is electrolyte protection. Palladium as a metal does not corrode in an acidic environment, whereas silver and the copper that it is alloyed with does. If the palladium step is skipped, silver and copper will eventually disrupt the final quality deposited as their presence increase with time in the rhodium bath in the form of metallic contamination.

The high-end fashion accessories sector uses a completely different finishing cycle to obtain a similar color. The principal reason being that the initial base materials are typically either brass or zinc alloy and require a different plating sequence to finish to a high-quality result.  The most commonly used cycles consist of 5-7 plating processes involving many layers for technical benefits. The selected sequence starts with alkaline copper for adhesion, followed by acid copper for surface brightness, moving to white bronze for improved hardness, then to palladium for oxidation resistance, and finally 1-2 layers of ruthenium. In the case 2 ruthenium layers are used, the first layer is light grey ruthenium which is then followed by black ruthenium, improving the overall products wear ability in the end.   

To incorporate the black gold alloy into the selected plating cycles would allow for the addition of the new layer to be measured using the two industries most standard processes as the qualitative benchmark. 

Black Gold jewelry plating sequence

In the case of the proposed changes to the jewelry process cycle, the black gold was selected to replace palladium as the technical layer given the black gold alloy itself contains a high percentage of palladium along with the additional fact that both electrolytes have a similar alkaline pH would mean minor processing deviations.

Sterling silver .925 parts were processed directly with 0.3 micron of the black gold alloy followed by 0.2 micron of black rhodium. Additional parts were processed with 0.3 micron of palladium and 0.2 micron of black rhodium to be used as benchmark in abrasion and corrosion testing. Following the treatments, the samples processed using the black gold as an under layer had a visibly darker color that those processed with palladium, even though the final finish was the same black rhodium processed using the same parameters. This process defined in this paper as process sequence 1.

Figure 2 - Top row: Plating sequence chosen as reference cycle; bottom row: Test cycle with the implementation of black gold.

Black Gold fashion accessory plating sequence

The selected changes to the fashion accessory process cycle were different given the complexities of the plating process itself. In this case, we focused on the finishing cycle which incorporates 1 layer of ruthenium in the sequence. The black gold deposit was tested as a technical layer by using it to replace the palladium layer in order to keep a similar cost, making quantitative room for the black gold. This process defined in this paper as process sequence 2.

Figure 3 - Top row: Plating sequence chosen as reference cycle; bottom row: Test cycle with the implementation of black gold.

In addition, the black gold was implemented into the process as a final finish given that it shares a similar color to gunmetal grey ruthenium. As both deposits have an L coordinate of 58, this would be used to benchmark the black golds resistance to oxidation with a market segment standard.

Figure 4 - Test cycle with the implementation of black gold as a final layer to test in contrast to ruthenium.

Brass parts were processed both with standard market segment plating sequences, and the relative black gold process deviations for qualitative testing.  This process defined in this paper as process sequence 3. 

Comparative testing

On the basis of the above information, the parts processed with more common plating cycles from the two selected market segments were comparatively tested against those processed with the black gold layer positioned in the sequence. Standardized normative testing methods were used to simulate oxidation and abrasion.

Abrasion resistance

The test method used to gauge abrasion resistance in the high fashion accessory market is the Turbula test, designed to simulate wear or abrasion. A specific machine is used in which the rotation is mounted on a pivot creating a more aggressive environment compared to a standard tumbler.

The media used is elongated pyramid shaped ceramic, with a distinct form granting both sharp and flat contact points simulating two aggressive forms of abrasion. Testing is preformed with the media weighing roughly 41 grams per 100 pieces. Five parts are treated each test, with a fixed evaluation phase following a turbula cycle at a fixed time and speed of 72 RPM.

Jewelry sequence results

Following one 3-minute abrasion cycle, the .925 silver specimens which utilized the black gold deposit as an intermediate layer proved to be more resistant to abrasion than the parts treated with the traditional palladium process. Further evaluation determined a better bonding between the black gold and black rhodium layer when compared to traditional plating methods.

Figure 8 - Top row: palladium as an intermediate layer; bottom row: black gold as an intermediate layer both after abrasion cycle

  

Fashion sequence results

Following one 30-minute abrasion cycle, the brass samples which utilized the black gold deposit as an intermediate layer proved to be more resistant to abrasion than the parts treated with the traditional palladium process which demonstrated complete loss of the ruthenium layer and a failed result.

Figure 9 - Left: black gold as an intermediate layer; right -  palladium as an intermediate layer both after 30 minute abrasion cycle 

Synthetic sweat resistance

The synthetic sweat normative the high-end accessories perform is NFS 80-772:2010-10 which is a direct contact test and more aggressive than atmospheric versions of the test. Following this normative, samples are put into direct contact with an absorbent felt which has been doused with an artificial sweat solution. The sample is sealed air tight and held at a consistent temperature of 55⁰C. The samples are then held for a predetermined duration of time in increments of 24 hours.

Following one 24-hour synthetic sweat cycle, the .925 silver samples which employed the black gold deposit as the middle layer demonstrated superior resistance compared to the parts treated with the customary palladium procedure. The surface of the pieces using the palladium process revealed elaborate signs of chemical aggression.

Figure 9 - Top row: palladium as an intermediate layer; bottom row: black gold as an intermediate layer both following 24-hour synthetic sweat cycle.

Salt spray

Following the ISO normative 9227, objects to be tested are suspended in a sealed chamber and submitted to a constant salt spray for a predetermined duration of time. The test is designed to simulate a corrosive environment and test the substrates resistance in a cycle time 96 hour.

To test the salt spray resistance of the black gold alloy, process sequence 3 was used because in this case the black gold alloy is exposed as the final layer.

Following one 96 hour salt spray cycle, the brass  samples finished with both ruthenium and the black gold alloy showed no signs of corrosion.

Figure 9 - Left: Black Gold as top layer; Right:: Ruthenium as top layer both following 96 hour salt spray cycle

Conclusions

Initial laboratory testing has shown that the black gold alloy improved qualitative results when used as a technical/intermediate layer most notably, synthetic sweat resistance when applied over silver as a substitute for palladium and finished with black rhodium. When tested as a final layer, the corrosion resistance was similar to that of ruthenium, but given the large cost differences when comparing palladium and gold with that of ruthenium and paired with the fact that the two electrolytes yield the same color, the use in this sense is unlikely.

The fluctuating pH of the black gold electrolyte can be seen as a limitation as it restricts the products use to qualified plating factories as it cannot be used in beaker plating which is quite common in the jewelry sector.

 Testing will continue into the future as the black gold electrolyte offers a wide range of obtainable thicknesses and considering the many different process sequences used throughout the decorative finishing industry, open up many doors of opportunity for this process, if combining electroplating with other technologies such as PVD continues to gain popularity.

In the end, enhanced abrasion and corrosion resistance will remain a central target for many companies in the surface finishing sector. Qualitative improvements to commercialized goods improve customer satisfaction, increase brand reputation, and most importantly decrease our overall footprint to the ecosystem with longer lasting products.

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