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A CASE STUDY OF LOW PHOSPHORUS ELECTROLESS NICKEL AS A REPLACEMENT FOR CHROMIUM
Michael Conti - The ARO Corporation Peter J. Vignati
Fidelity Chemical Products Corporation Newark, New Jersey
Presented at: FINISHING '95 CONFERENCE AND EXPOSITION
September 18-21, 1995 Cincinnati, Ohio
INTRODUCTION The ARO Corporation based in Bryan, Ohio is part of worldwide Ingersoll-Rand and manufactures industrial fluid power and fluid handling equipment of world class design, function and durability. Their piston and diaphragm pumps are air operated, of rugged construction, and their broad material compatibility insure long, dependable service life. The pumps are used in a variety of applications such as oil reclamation, material transfer, water treatment, assembly, packag- ing, cutting oil, coolant, lubrication, waste treatment, bulk storage and surface finishing.
Several years ago ARO began to investigate alternative materials and coatings to chromium plating for their line of reciprocating pumps. One of the reasons that ARO started to evaluate alternatives to chromium plating was that there was a definite trend in the finishing market towards more corrosive and more abrasive coatings. Environmental regula- tory pressures on industry to reduce VOC forced the development of more waterborne, two-component and high-solids materials. Many applications have solids suspended in the fluids resulting in increased abrasive wear and corrosion on components.
One coating that ARO knew that they were going to evaluate as a potential replacement for chromium was electroless - nickel. Fidelity Chemical Products Corp., based in Newark, NJ was contacted because they are the leading supplier of proprietary electroless nickel chemistry in the United States today. Fidelity has been developing specialty electroless nickel plating solutions for over 15 years and has taken a leadership position in developing electroless nickel coatings
that produced a smooth and adherent electroless nickel coating suitable for ARO products. Many samples were run in Fidelity’s Pilot Shop facility to demonstrate the coating and optimize the plating process.
for engineering applications. ARO and Fidelity engineers worked together to develop a pretreatment and plating process -
There are many types of electroless nickel solutions on the market today and most of them deposit an alloy of nickel and phosphorus. The major types of electroless nickel coatings are outlined below:
TYPE PHOSPHORUS o/u WT HARDNESS(HVl00) AS PLATED Low Phosphorus 1-4% 625-675 Mid Phosphorus 5-9% 450-500 High Phosphorus 10 and above 450-500
ARO chose the low phosphorus electroless nickel coating (Fidelity 4008), for their evaluation since it has a much higher as-plated hardness than the other two types of electroless nickel coatings. This increased hardness allows the low phosphorus coating to perform extremely well in abrasive and erosive wear situations such as those encountered in a diaphragm pump.
OBJECTIVE The objective of the entire investigation was to develop wear and corrosion resistant components that provide an operational surface with corrosion resistance equal to or better than 3 16 stainless steel and with wear resistance superior to hardened 440C with hard chromium plating. The alternative coating process to chromium plating also had to be cost effective and part of an overall robust manufacturing process.
Another reason that chromium replacements were evaluated at ARO is that hexavalent chromium plating is a hazardous process. The hexavalent form of chromium is highly toxic and strongly suspected of causing lung cancer. Chromium electroplating and anodizing tanks are some of the largest sources of airborne chromium emissions. The Clean Air Act, as amended in 1990, directed the U.S. Environmental Protection Agency(EPA) to regulate emissions, including chro- mium compounds, from a wide range of industrial sources. ARO knew that the EPA would be tightening regulations on chromium emissions and in November of 1994, the EPA issued final national regulations to control air emissions from chromium electroplating and anodizing tanks. ARO felt that there was a strong need to eliminate chromium plating in its facility and bring a process in house that was not only more environmentally friendly than chromium plating, but one that produced a coating that was superior to chromium in its ability to withstand abrasive wear and corrosion.
MARKET PERCEPTION ARO was fully aware that their major customers viewed hard chromium over stainless steel as the premier material for pumps. The perception in the marketplace is that chromium plating and passivation of stainless steel are the answers to all corrosion and wear problems. This perception needed to be addressed if ARO was to be successful with the coating change they were contemplating. It was hoped that if ARO were to come up with a suitable replacement for chromium that it would show itself to be a technological market leader as well as highlight its global awareness of environmental issues.
LABORATORY TESTING In Phase I, a total of twelve different coatinghbstrate combinations were laboratory tested in order to rank possible alternatives to chromium plating. Low Phosphorus electroless nickel was not laboratory tested at this time. but would be included in the Field Testing phase of the evaluation. Chromium plated specimens were also examined in this study and provided a baseline from which to improve upon. It was assumed that one coating may not replace chromium in all applications, but that each individual coating application needed to be judged on its own merits.
Three types of substrates were investigated; 17-4PH, a precipitation hardenable stainless steel, AIS1 3 16. an austenitic stainless steel: and AIS1 101 8, a plain carbon steel in the through-hardened condition. The twelve surface treatment1 substrate combinations were:
COATING SUBSTRATE Uncoated 17-4PH, 101 8 , 3 16SS Hard Chromium(O.00 1 ”) 17-4 PH Hard Chromium(0.003”) 17-4 PH Hard Chromium(O.006”) 17-4 PH HardCor 316 SS Titanium Nitride 1018 Chromium Nitride 1018 Ion Nitriding 1018 Diamond Black Boron Carbide 1018 Tungsten Carbide 17-4PH Non-metallic Polymer A 1018 Non-metallic Polymer B 1018
For a description of the above processes see Appendix A.
WEAR TEST Wear tests were conducted on a Falex Ring & Block Friction & Wear testing machine. The Timken-type tapered roller bearing wear rings were run against conforming ultra high molecular weight polyethylene blocks. (This is a typical packing material that a piston rod would come in contact with). Physical test parameters including the use of a slurry, oscillation speed of the rings, and loading were carried out in such a manner as to simulate actual service conditions. Ring weight change, block weight change and friction force were tabulated and are graphically depicted in Exhibit 11, 111, and IV, respectively.
AVERAGE RING WEIGHT LOSS vs. SURFACE TREATMENT 6
As shown in Exhibit 11, the weight loss varied with surface treatment. However, consideration must be given to the variations in the densities of the surface treatments. For the same weight loss, a higher density material would lose less mass than a lower density material. The chromium nitride and Diamond Black boron carbide coatings had lower average weight losses than other surface treatments, and substantially lower than the chromium plating.
AVERAGE WEIGHT CHANGE vs. SURFACE TREATMENT 2 , I 1 0 -1 -2
~ C T D 1 , O O , ” Cr ’ 003’’ Cr ’ oo6” Cr’ HRDCR’ TiN I
RING WEIGHT CHANGE BLOCK WEIGHT CHANGE
Exhibit I11 includes the change in block weight. Some blocks gained weight and some blocks lost weight, depending on the substratehrface treatment combination. The thicker chromium plating had block weight gain and the highest value of block weight loss was in the uncoated 17-4 PH sample.
FRICTION FORCE vs. SURFACE TREATMENT
I 4 , I
SURFACE TREATMENT I The friction forces shown in Exhibit IV are a measure of the torque between the wear ring and the conforming block under the operating conditions. The friction force was measured at the beginning, middle and efid of the wear cycle (average values are shown). The tungsten carbide coating had the highest average friction force while the non-metallic polymer coatings had relatively low measured friction forces.
Some general observations on the wear data in the laboratory test conditions are: 1.
combinations investigated. 2.
ments in wear resistance. 3. 4. After the wear test each ring was sectioned transverse to the surface treatment in both the wear scar and unworn areas and examined.
The chromium plated specimens experienced the greatest weight loss of the 12 surface treatmenthubstrate
Increasing the chromium plating thickness from 0.003” to 0.006” did not result in any observable improve-
The tungsten carbide had the highest coefficient of friction of all the conditions tested. The very thin boron carbide coating was worn away during the test, revealing the base material.
U CORROSION TEST After the wear test, the rings were subjected to ASTM B-117 neutral salt spray testing. The wear rings were removed from the salt spray chamber and photographed every 24 hours. Since all coatings examined protect the substrate by barrier coat protection, this test measured the porosity of each individual coating.
The two different coatings that performed the best in this evaluation were non-metallic Polymer A and non-metallic Polymer B. It was evident that in order to adequately protect a non-stainless substrate such as AIS1 10 18 from corrosion and wear damage, a coating must be hard and wear resistant and contain no voids, scratches or other discontinuities which would expose the substrate and enable corrosion to occur.
FIELD TEST Based on the laboratory evaluations of wear and corrosion protection ability, four surface treatments were deemed worthy to merit further investigation and proceed to the Field Test stage. (1) The HardCor process: (2) Non-metallic Polymer A; (3) Multi-Arc PVD Chromium Nitride; (4) Fidelity 4008 low phosphorus electroless nickel plating process.
Actual Field Testing proceeded for a period of approximately 18 months. A variety of different pump models and test sites were chosen in order to compile sufficient data to arrive at a qualified conclusion. Multiple coatings including the standard hard chromium were tested at each site for comparison purposes. Visits to each test site were done by a Project Team member on a bi-monthly basis.
Visual observations were taken to check the condition of the coated pilot rods and to check for O-ring wear. Actual wear of the coatings evaluated were measured after 1-2 million cycle intervals and reported. All of the coatings failed in at least one of the Field Test sites except the Fidelity 4008 low phosphorus electroless nickel. This coating actually showed less wear and corrosion than the chromium plated standard. For this reason, ARO decided to specify Fidelity 4008 low phosphorus electroless nickel.
ELECTROLESS NICKEL VERSUS CHROMIUM Electroless nickel coatings are alloys of nickel and phosphorus produced by autocatalytic chemical reduction with hypophosphite. The chemical and physical properties of the deposit vary primarily with its phosphorus content and subsequent heat treatment. The chemical makeup of the plating bath can also affect the porosity and corrosion resistance of the deposit. Electroless nickel deposits can be specified by following ASTM B733, Mil C-26074, or AMS 2405.
Electroless processes produce coatings of uniform thickness on irregular shaped parts, provided the plating solution circulates freely over their surfaces. This is in contrast to electrolytic processes, such as chromium plating, which deposit thicker deposits on edges of parts and less thickness of plate in recessed areas. Inside diameters require no special fixturing with electroless nickel and the deposit is uniform in thickness from the outside diameter to the inside diameter. This is an important consideration because ARO found that they could plate electroless nickel “to size” within 0.0001”, and could eliminate any post grinding of chromium plating that was necessary to achieve uniform thickness over the entire piece. When chromium is ground improperly it can produce cracks in the chromium surface that can propagate into the substrate causing failures.
Electroless plated parts can function better in service because they often fit better than chromium plated parts. If a part requires chromium plating the engineer will normally allow the chromium plater sufficient tolerance to compensate for the size variation that typically occurs with an electroplated product. If the engineer can specify electroless nickel as a replacement he can also reduce the allowable ioieiance. Geiieraiiy, pai-ts piateb with electi-oiess iiickei have a iiioi-e consistent finished size than parts plated with chromium.
ELECTROLESS ELECTROLYTIC UNIFORM COATING THICKNESS NON-UNIFORM COATING THICKNESS
Electroless nickel coatings have multifunctional properties such as high hardness, heat hardenability, abrasive wear and corrosion resistance, can provide a diffusion barrier, and can provide a solderable surface. The coatings are character- ized as amorphous, or "glass-like" in structure versus chromium coatings which are crystalline and micro-cracked. Electroless nickel phosphorus coatings contain 1 - 14% by weight phosphorus.
I ELECTROLESS NICKEL
Face on view 20x CHROMIUM Face on view 20x
The coating chosen to replace chromium at ARO is Fidelity 4008, a low phosphorus coating with 1-4% by weight phosphorus. These low phosphorus coatings are typically the hardest and most wear resistant of all the electroless nickel phosphorus coatings. It has the following deposit properties as compared to chromium:
LOW PHOSPHORUS EN CHROMIUM 625-675 950- 1050
Melting Point ("C) 1050-1 150 1850-1 890 Hardness (HV100)
DEPOSIT CORROSION RESISTANCE
ASTM B 1 17 Salt Spray Steel Substrate 600
HOURS TO FAILURE
200 ! I
0.5 1.5 DEPOSIT THICKNESS (mils)
10 10 CARBON STEEL HARD CHROME 0 LOW PEN
One of the most important differences between electroless nickel and chromium coatings is their corrosion resistance. Both are barrier coatings and protect the base material by sealing it off from the environment rather than by sacrificial protection. In most environments the corrosion resistance of hard chromium is much less than that of electroless nickel.
Due to the cracks present even in thick chromium deposits, chromium coatings offer only limited corrosion protection. These cracks often offer pathways through the coating for a corrosive environment to reach the base material. Chro- mium is rapidly attacked by reducing environments and is subject to pitting and localized attack in halogens, especially oxidizing halogens such as ferric chloride. Chromium coatings will often develop a network of rust spots across their surface.
Electroless nickel due to their amorphous structure can provide true barrier coat protection to the substrate. They do not offer any pathways to the substrate as with chromium coatings. The corrosion resistance of electroless nickel is similar to that of other high-nickel alloys. The coatings are resistant to alkalis, seawater, acid gas environments, and have found widespread use in the oil, gas and chexical proccss industries.
I I TABER WEAR INDEX I
mg wt. loss / 1000 cycles
Even though low phosphorus electroless nickel is not as hard a coating as chromium it can perform as well, or better, in wear applications. It is a misconception that the hardest part will always provide the best wear. It is necessary to investigate the tribological phenomena that occur between mating parts when selecting materials to optimize the “wear pair”.
Surface finish plays an important role in deposit wear resistance and corrosion resistance of coatings. Electroless nickel will essentially “mirror” the surface onto which it is being plated. It will not fill in any voids or high spots and has no ability to level as in an electroplated nickel deposit. Chromium, likewise, will also replicate the surface onto which it is being plated, yet has more of a tendency to produce nodules than electroless nickel. These nodules can be the source of premature wear on the bushing or packing material it is wearing against and enhance the potential for galling.
SUMMARY Some of the benefits that ARO saw when they switched coatings to the Fidelity 4008 low phosphorus electroless nickel were:
1. 2. 3. 4. 5 .
Corrosion and wear performance equivalent to or better than chromium. No post grinding after electroless nickel plate (chromium needed to be ground). Electroless nickel is a less toxic process to operate than hexavalent chromium plating. Electroless nickel is a cost effective process. Electroless nickel is a robust and easily repeatable process.
A cut-out ofa . pump showing a plated piston. A typical electroless nickel plating line.
9 endix A Description of Processes
HardCor - Is an anti-wear and anti-corrosion surface treatment that requires an austenitic stainless steel (AIS1 300 series). The hardened layer produced in the HardCor process is relatively shallow. HardCor has a 300 C maximum application temperature and is reported to be less expensive than physical vapor deposistion (PVD) or chemical vapor deposition (CVD) applied surface treatments.
Polymer Coatings - These coatings provide lubrication, control friction, and increase wear and heat resistance. This is accomplished by curing a fluoropolymer (PTFE) to the substrate at a low temperature.
Arc-Vapor Deposited Coatings - The arc evaporation system provides higher ionization levels, resulting in higher deposition rates and lower processing temperatures. It modifies the standard PVD process by adding a cathodic arc source.
Ion Assisted Chromium Nitriding - In a two step process, a very thin film of the material to be ion mixed is deposited on the surface of the component. Then an energetic ion beam of chemically reactive nitrogen is used to mix the depos- ited film on the surface of the target. Ion beam mixing requires one-of-a-kind custom designed units. In some instances the process can substitute for conventional nitriding treatments. However, the wear mechanism must be abrasive or adhesive and the wear surface must not be heavily loaded.
Diamond Black - This is a hard boron carbide alloy coating applied by a low-temperature deposition process. It is extremely thin, thus causing no alteration to heat treated tools or warpage of precision tools. The ultra smooth, low friction coating provides better finishes and mold release to tooling.
Duncan, Ronald, “Electroless Nickel: Alternative to Chromium Coatings”, Metal Progress, June 1985.
Riedel, Wolfgang, “Electroless Nickel Plating”, Finishing Publications Ltd., 199 1.
Gawrilov, G.G., “Chemical Nickel Plating”, Portcullis Press, 1979
Jeanmenne, Robert, Caterpillar, Inc., “It Makes ‘Cents’ to Substitute Electroless Nickel for Hard Chromium Plating”, Electroless Nickel ‘89, 1989