Trivalent yellows deliver process benefits, design flexibility
The search for trivalent chromates complying with the European Union’s End of Life Vehicle (ELV) and Restriction of Hazardous Substances (RoHS) directives has emphasized trivalent chromates that are clear/blue and black. Yellow has been largely left behind for automotive applications because of the challenges involved in achieving an acceptable yellow color, and because the performance of the trivalent clear/blue chromates in salt spray corrosion testing equals or exceeds that of hexavalent yellow chromates. Yet another challenge with existing technology for trivalent yellow chromate conversion coatings is that multiple dips are necessary to achieve the level of corrosion protection required.
All these things are beginning to change. This article will discuss two new processes available for the zinc electroplater and how they address these challenges to give automotive designers a yellow trivalent chromate option.
Hex Yellow Chromates
Hexavalent yellow chromates historically used in automotive, electronics and other applications consist of a blend of acids, catalysts, and chromic acid formulated to produce an iridescent yellow finish. This finish has been a true workhorse for the zinc metal finishing industry, typically achieving 72–144 hr of salt spray resistance.
Hex yellow chromate is a thick film with self-healing characteristics, which means it doesn’t lose corrosion protection when scratched. This is because the yellow chromate film is gelatinous and contains water of hydration. The nature of the film also means that it will dry out and crack above 150°F, becoming hard and losing its salt spray corrosion protection.
This presents difficulties if parts have to be baked to release any hydrogen that may be absorbed during the zinc electroplating process. A typical baking cycle is 4–24 hr at 325–400°F, far above the temperature at which the yellow chromate film would dry out and lose its corrosion resistance. Therefore, the procedure for hexavalent yellow zinc chromate requires that zinc-plated parts be taken off the line, baked, then placed back on the line for chromating.
This processing paradigm has changed with use of new trivalent yellow chromate formulations, which provide an ELV- and RoHS-compliant option for end users who still prefer yellow chromate finishes for color identification or other purposes.
EPI, Electrochemical Products, Inc. has developed two trivalent yellow formulations. The first, called E-Chrome Ultra Yellow, is a two-part trivalent chromate that provides up to 150 hr of salt spray resistance to white corrosion without a topcoat. It does not rub off and it will pass the thumb test. The product also offers good UV protection and will not fade in the sun.
The product results in a conversion coating that is iridescent yellow with a slight green/red hue. About half of acid chloride zinc, alkaline non-cyanide zinc, and zinc cyanide electroplaters accept the color. Some electroplaters that do not accept the color object to it being too green, and this resulted in development of E-Chrome Super Yellow, which provides a yellow coating containing less green.
The Ultra Yellow product is made up of 3% by volume of component A, 2% by volume
Of component B, and 95% tap water. Ammonium hydroxide is used to adjust pH to 1.8. The process runs at a range of temperatures from 80–140°F. Optimum immersion time is 60 sec, but parts can be immersed for 30–120 sec.
After electroplating to a minimum thickness of 0.0003 inch using an acid chloride, alkaline non-cyanide, or cyanide zinc plating process, parts must be rinsed and then bright dipped using sulfuric acid at a concentration of 0.5–1.0% by volume. Nitric acid bright dips can cause a poor yellow chromate film. After a rinse, the trivalent chromate is applied and parts are rinsed a third time. Parts can be dried in an oven at temperatures in excess of 150°F, then baked to avoid hydrogen embrittlement if required.
The resulting conversion coating averages 150 hr to 5% white corrosion in salt-spray testing. If these same parts are baked for at least 2–4 hours, salt-spray corrosion resistance increases to 300 hr to 5% white corrosion. The mechanism of this phenomenon is still being studied, but we believe that the trivalent chromate, corrosion inhibitors and coloring agents react when heated to form a film that offers superior corrosion protection.
What makes this process different from other trivalent chromates? The color agents have an additional benefit which will be discussed further in this paper.
At lower temperatures, the chromate achieves 24–72 hr of salt spray corrosion protection. At 90–110°F, it can withstand 48–120 hr, and at 115–140°F, 150 hr.
Bath pH also plays an important factor in corrosion protection of the finished conversion coating. The operating pH range is 1.6–2.0, with 1.8 being optimum. Operated at pH of 1.0–1.6, the coating provides less corrosion protection. Users can raise pH by addition of ammonium hydroxide and lower it using the A component of the bath.
This trivalent yellow chromate also offers very good UV resistance relative to some other dyed trivalent chromates, which fade in a matter of days in the sun. The color agent chemistry differs from standard organic dyes, resulting in color-fastness that approaches that of a hexavalent yellow chromate.
Testing was done using a UV fluorescent lamp at wavelength of 300–400 nm for 500 hr. A zinc-plated panel finished using the trivalent chromate was compared with a zinc-plated panel with hexavalent yellow chromate. The results after 500 hr were no fading with either process, meaning that E Chrome Ultra Yellow had performance similar to that of the hexavalent chromate process.
|Table 1: Properties Comparison of Trivalent Chromate Processes|
|Characteristic||Ultra Yellow||Super Yellow|
|Color||Yellow-green iridescence||Yellow iridescence|
|Salt-spray protection, hr||150 (300 if baked)||180–200|
|Thickness, nm at 95%
Zn by weight
|30 sec: 2057||30 sec: 2092|
|60 sec: 3088||60 sec: 2470|
|Number of tanks||One||One|
|Bakeable?||Yes, and increases corrosion resistance||Yes, does not increase corrosion resistance|
|Relative cost||Most Expensive||Least Expensive|
|Wipes Off With Thumb?||No||No|
With initial UV resistance testing done, a second test was performed by Q-Lab Corp. (Cleveland, OH) using its Q-Sun test method. This technique gives more “real world” results than a single-wavelength test because it includes visible wavelengths, to which all bright colors are sensitive). The test apparatus, a xenon arc test chamber, complies with the ASTM G 155 test method and covers a spectrum from 295–800 nm.
After 100 hr in the Q-Sun test, both trivalent yellow coatings faded to a blue color. The hexavalent yellow process faded as well but still showed some yellow. Our company is doing more work to develop a trivalent process that can better withstand the Q-Sun test.
Besides a slightly green color cast, E Chrome Ultra Yellow does have other two challenges. The first is cost—approximately 10× more for make-up costs than a hexavalent yellow. The second is that it does not have the self healing characteristics of a hexavalent yellow chromate. But, if you are a rack zinc electroplater who bakes for hydrogen embrittlement, your cost savings will likely negate the added make-up cost of the process.
Another Tri Alternative
Attempting to address the high make-up costs, greenish color cast and reduced scratch resistance of the process just described, our company developed another trivalent chromate formulation called E-Chrome Super Yellow. Make-up costs for the process are 30–35% less than the make-up costs of the Ultra Yellow, and some zinc electroplaters like the color better. For barrel plating, Super Yellow wins hands-down, because it produces a trivalent chromate film that offers moderate scratch resistance.
Bath composition is similar to that previously described, consisting of 3% by volume of Component A, 2% by volume of Component B and 95 tap water. Bath pH requires no adjustment, and the bath operates over a temperature range of 80–140°F. Parts are immersed for 30 sec–5 min, with 60 sec being optimal. Processing is identical to the Ultra Yellow process.
In salt-spray testing, parts coated with this second trivalent chromate averaged 180–200 hr to 5% white rust. Baking of coated parts did not affect salt-spray corrosion protection, nor did the color of the film change during baking process.