by Burhanuddin Kagajwala, Timothy D. Hall, Maria Inman and E.J. Taylor (Faraday Technology Inc.); Bruce Griffin (The Boeing Co.); George Cushnie and Randal Taylor (Advanced Tooling Corp.); and Mark Jaworowski and Joe Bonivel (United Technologies Research Center)
This paper is a peer-reviewed, edited and updated version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
This paper discusses recent research work on the development of a functional trivalent chromium plating process from a trivalent-based electrolyte to replace hexavalent chromium plating. Hexavalent chromium plating has been used for many years to provide hard, durable coatings with excellent wear and corrosion resistance properties. However, hexavalent chromium baths have come under increasing scrutiny due to the toxic nature of the bath, effects on the environment and workers’ health. In this paper are results from our development program aimed at achieving properties comparable to existing hexavalent chromium plating for functional applications. Specifically, recent efforts in plating chromium on the internal surfaces of cylindrical parts will be presented, as well as wear test data.
Keywords: hexavalent chromium replacement, trivalent chromium electroplating, functional chromium, wear testing
Our work continues to address the need for a drop-in replacement chromium plating process that can coat complex, hard-to-access surfaces such as the interior of landing gear, using an environmentally benign trivalent chromium bath. We have developed a green manufacturing process that meets the stated EPA needs by improving an existing process while utilizing a novel green approach that eliminates worker exposure to the carcinogenic hexavalent chromium bath.
The USEPA identified hexavalent chromium as one of 17 “high-priority” toxic chemicals based on their known health and environmental effects, production volume and potential for worker exposure1 which were targeted for 50% reduction by 1995.2 The urgency of this issue was further underscored by a recent memorandum from the Under Secretary of Defense, referring to the need to minimize or eliminate the use of hexavalent chromium as an “extraordinary situation,” requiring the government and industry to “go beyond established hazardous materials management processes” and “more aggressively mitigate the unique risks to operations now posed by hexavalent chromium.”
Current wear resistant coating solutions include alternative technologies like high velocity oxyfuel (HVOF) or new platable material systems such as cobalt-phosphorus alloys. HVOF technology is limited by the geometry of the part, requiring that the regions that need to be coated are non-complex and be located within line-of-sight to the spray head. Additionally, HVOF systems are extremely expensive to implement at industrial and governmental depot facilities. As for new platable material systems, they require the use of potentially toxic materials including nickel and cobalt,3 which can create manufacturing and environmental problems due the need for new process specification sheets and environmental exposure initiatives. The advantage of our approach is centered on developing a drop-in replacement using an environmentally benign trivalent chromium electrolyte and controlling the deposition process through use of sophisticated waveforms engineered to deliver the desired chromium coating properties rather than the use of novel material systems or carcinogenic hexavalent chromium plating technologies. Additional advantages to the additive-free trivalent chromium plating process relative to hexavalent chromium plating include:
1. Trivalent chromium is non-toxic, non-hazardous and is not an oxidizer. Therefore, meeting air quality regulations is easier and working conditions are greatly improved. The exposure limit for trivalent chromium is an order of magnitude higher than that of hexavalent chromium.
2. Disposal costs are significantly reduced for trivalent chromium plating. Hydroxide sludge generation is reduced ten to twenty times because trivalent chromium generally operates at a chromium content of about 4-20 g/L vs. 150-300 g/L for a hexavalent bath.
3. As there are no proprietary additives in the trivalent bath, the rinse water may be recycled.
In addition, trivalent chromium plating has the following technical advantages:
1. The trivalent chromium-plating bath is not sensitive to current interruptions.4 Therefore, the innovative approach used in this program is more suitable for trivalent chromium plating than for hexavalent chromium plating.
2. Drag-in of chloride and sulfate from previous nickel-plating operations into the trivalent chromium process is tolerated.5 By contrast, chloride and sulfate drag-in upset the catalyst balance in a hexavalent process.
3. Throwing power for trivalent chromium plating, which is poor in a hexavalent chromium bath, is good and similar to other metals such as copper.5
Therefore, trivalent chromium plating has numerous environmental, health and technical advantages relative to hexavalent plating. Faraday is uniquely positioned to undertake the development of trivalent chromium plating for the applications stated above due to its years of experience with the science of applying pulse and pulse reverse waveforms to the electrodeposition of functional and decorative chromium coatings. For example, with prior funding from the USEPA SBIR program, Faraday demonstrated the feasibility of thick, hard functional chromium deposition from a trivalent chromium plating bath onto the exterior surface of rods and coupons (Contract #68D50116), and then built upon that work to develop a cost-competitive plating bath for those parts (Contract #68D00274). In a current program with the National Center for Manufacturing Sciences (NCMS - Contract #140427), we are validating our electrochemical process for simple line-of-sight applications by depositing a chromium coating onto flat coupons that passes the accepted functional property standards set by the aerospace community, including adhesion, hardness and porosity. To date, Faraday has demonstrated that the chromium coatings prepared using our process have functional properties equivalent to the coatings produced with a hexavalent chromium bath (Table 1). These data demonstrate equivalent or superior: (1) plating rate, (2) Knoop hardness, (3) current efficiency, (4) hydrogen embrittlement behavior, (5) adhesion, (6) corrosion resistance, (7) porosity, (8) thickness, (9) Taber Abrasion, Ball on Flat Reciprocating and Dithering wear resistance and (10) no hexavalent chromium formation over a 1400 A-hr processing window.
- Physical property comparison between chromium deposits produced from a current hexavalent chromium process and the Faraday trivalent chromium process.
| | Faraday’s Trivalent Chromium Plating
| |Thickness (per AMS 2460, 3.4.1) | |Comparable to hexavalent chromium plating.
| |Knoop hardness (per AMS 2460, 3.4.3) | |Comparable or superior performance to hexavalent chromium plating (800-1000 KHN; average 947 KHN)
| |Hydrogen embrittlement (per ASTM F519 1a.1) | |Comparable performance to hexavalent chromium plating.
| |Porosity (per AMS 2460, 3.4.4) | |Comparable performance to hexavalent chromium plating.
| |Adhesion (per ASTM B 571) | |Comparable performance to a baked hexavalent chromium deposit.
| |Corrosion resistance (ASTM B117) | |Comparable performance to a baked hexavalent chromium deposit.
| || |3.5 mil/hr compared to 1 mil/hr.
| || |42% compared to 15% for hexavalent chromium plating.
| |Hexavalent chromium formation | |After 1400 A-hr, no observed Cr+6 formation.
| |Taber abrasion test (ASTM D4060) | |Comparable performance to a baked hexavalent chromium deposit.
| |Reciprocating ball-on-flat (ASTM G133) | |Comparable performance to a baked hexavalent chromium deposit.
| |Oscillation (Dithering wear test) | |Comparable performance to a baked hexavalent chromium deposit.
These data demonstrate the feasibility of the process and provide the basis for further technical qualification and prototype design. The primary hurdle being discussed in this paper is the demonstration and development of our chromium electrodeposition process for hard to access, complex shapes, such as the interior of cylindrical shafts.
Our chromium plating process utilizes pulse and pulse reverse waveforms for trivalent chromium plating. Figure 1 is an example of such a waveform, consisting of a cathodic (forward) pulse followed by an anodic (reverse) pulse and a relaxation period (off-time). The cathodic peak current (ic), cathodic on-time (tc), anodic peak current (ia), anodic on-time (ta), and the relaxation-time (to) are individual variables for process control. The sum of the cathodic on-time, anodic on-time and relaxation-time is the period of the modulation and the inverse of the period is the frequency. The cathodic duty cycle (γc) is the ratio of the cathodic on-time to the period, and the ratio of the anodic on-time to the period is the anodic duty cycle (γa). The frequency and duty cycles are additional variables for process control. The average current density (iaver) or electrodeposition rate is given by iaver = icγc – iaγa.