Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Aircraft Applications for Electroless Nickel Plate
THE ENGINEERING USE of electroless nickel plate is in-creasing rapidly in the aircraft industry. The purpose of this paper is to highlight the properties of electroless nickel as related to applications suitable for aircraft. It deals with the benefits from special properties and points out pitfalls and dangers of misapplication of electroless nickel deposits. It is directed to design engineers and applications engineers with the hope that a better understanding of what electroless nickel plate can and cannot do will lead to its better use.
Electroless nickel plating is a method of continuously depositing, by catalytic reduction, a homogeneous coating of nickel, nickel-phosphorus, nickel-boron alloy, or pure nickel, on a suitable basis metal. The source of electrical energy comes from within the plating solution, being supplied by a chemical reducing agent. A critical balance between spontaneous plating and a stable solution condition is maintained by complexing agents in the solution. When nickel, iron, or other catalytic metals are introduced into the solution, deposition takes place on that metal surface only. Plating continues because the deposited nickel is also catalytic to the reaction. It is said to be autocatalytic.
DESIRABLE PROPERTIES OF ELECTROLESS NICKEL PLATE
GOOD CORROSION RESISTANCE - The unique ability of electroless nickel to deposit a uniform thickness of nickel- phosphorus alloy over all surfaces provides superior corrosion resistance not possible with electroplated nickel coatings. Electroplating is limited by the laws of electrical conductance where current follows the path of least resistance. In most cases, this means areas on the device being plated closest to the anodes. The deposit then is thin or nonexistent in recessed areas, and excessively heavy on edges and sharp projections. On smooth surfaces, or controlled surface finishes, electroless nickel deposits have somewhat better corrosion resistance than electrodeposited nickel of the same thickness.
The uniform deposit produced by electroless nickel can eliminate the need
for finished machining, grinding, or lapping
operations. One can measure significant surfaces, such as a bearing race, before plating, and plate to the exact dimension required. Finishing is not required. To accomplish this consistently, it is important to use a solution capable of plating at a constant rate. It should be controlled by frequent and regular additions of the replenishment chemicals. Temperature and pH should be carefully controlled since these two factors have the greatest influence on plating rate. Test coupons can be plated, along with the parts, and tested occasionally with a micrometer. This further assures accurate control of dimensions.
HARDNESS AND LUBRICITY - The most frequent reason for selecting electroless nickel is its inherent hardness and lubricity. The as-plated hardness is from 400.700 Vickers hardness number. After heat treating, it increases to 900-1100 Vickers hardness number. Tabor abrasion tests indicate that electroless nickel plate is about two times more lubricating than electrodeposited Watts nickel.
DEPOSIT CHARACTERISTICS - Table 1 is a comparison of properties of various nickel systems.
FRICTIONAL PROPERTIES - The lubricity and smoothness of electroless nickel reduces galling tendencies; however, dissimilar metals are generally best for rubbing surface con-tact. The coating is easily wet by most organic liquids such as polyvinyl chloride and Teflon.* The frictional coefficient of various nickel-to-metal systems under non-lubricated and boundary-lubricated (chlorinated paraffin) conditions has been investigated. Results of some typical tests are given in Table 2.
Tests on the Falex wear machine have also shown heat- treated electroless nickel deposits to be at least equivalent in wear resistance to chromium deposits under rotating, lubricated conditions.
*E. I. DuPont Co. trademark.
Table 1 - Comparison of Properties of Various Nickel Systems
|As-plated hardness, Vickers||
|Purity of deposit, % nickel||
|Melting point, °F||1630.1640||
|Tensile strength, psi||
Table 2 - Frictional Coefficient between Electroless Nickel and Various Metals
System Un-lubricated Lubricated
Electroless nickel versus
steel 0.38 0.21
nickel Galling 0.26
chromium 0.45 0.30
electroless nickel 0.45 0.25
WHEN TO SELECT ELECTROLESS NICKEL - Electroless nickel should be selected for:
1. Salvage of bearing journals machined undersize.
2. Salvage of I.D. journals machined oversize.
3. Precision fit of mating parts.
Frequently, considerable cost can be saved by machining near the final finish and electroless nickel plating to the precise size required. Tolerances of 0.0002 in can be maintained.
CURRENT APPLICATIONS OF ELECTROLESS NICKEL
BEARING JOURNALS - Electroless nickel plate, heat treated to 65 Re, is used for bearing journals. The outstanding wear properties of the electroless nickel deposit caused this application to be considered. In production practice, it was found that plating to exact dimensions saved machining costs. Chrome- plated journals were previously used.
SERVO VALVES - The low surface friction and good corrosion resistance properties of electroless nickel make it ideal for plating servo valves. The electroless nickel deposit is more resistant to fluids used in the valves than is hard chrome or electroplated nickel. Further, the uniform deposit thickness of electroless nickel eliminates the finishing operations associated with electrodeposits.
TURBINE OR COMPRESSOR BLADES - Compressor blades are protected against the corrosive atmosphere passing over them by 0.001-0.003 in electroless nickel. This application is widely used in the aircraft industry. Some reasons for selecting electroless nickel for plating turbine blades are:
1. Deposit hardness can be increased by heat treating.
2. Deposit corrosion resistance is twice that of electrode- posited nickel of equal thickness.
3. Fatigue strength of the basis metal is reduced 25% less than when chromium plate is used.
4. The deposit is uniform, resulting in uniform corrosion resistance and consistent physical properties.
PISTON HEADS - Electroless nickel plate on aluminum piston heads results in increased wearability compared to chrome- plated aluminum piston heads.
CADMIUM PLATE REPLACEMENT - Cadmium plate in contact with aluminum sets up a galvanic cell, resulting in some corrosion. Electroless nickel is used to replace cadmium plate for parts which contact the aluminum skin of the aircraft.
SPLINES - Electroless nickel plate on the splined ends of hydraulic actuators results in increased wearability compared to chrome plate.
SEAL SNAPS AND SPACERS - Seal snaps and spacers are plated with 0.001 in of electroless nickel followed by cadmium plate, and heated to diffuse the cadmium into the electroless nickel deposit. This procedure saves many processing and ma-chining steps, while providing a superior product. In engine rebuilding, when a 0.001 in thick plate is not enough to compensate for wear, hard sulfamate nickel is first deposited, followed by a machining operation, if necessary. Then the electroless nickel-cadmium diffused process is employed.
OIL NOZZLES - Oil nozzle parts plated with electroless nickel to size result in a custom, accurate fit.
IMPELLERS - Bearing surfaces of impellers are plated with electroless nickel to improve wear characteristics and reduce machining time.
ENGINE MAIN SHAFTS - Electroless nickel plate on the main shaft of an engine provides a good bearing surface. When rebuilding is required, the electroless nickel can be stripped off and replated to the correct thickness. This is an advantage over chrome plate, which requires machining before replating.
PITOT TUBES - In pitot tubes, it is common to find parts made of dissimilar metals, such as electroformed pure copper, brass, silver solder, and stainless steel. Such metals in contact with each other set up galvanic actions causing corrosion. An electroless nickel plate on inside and outside critical surfaces not only prevents corrosion, but provides a uniform, wear- resistant surface.
HOT ZONE HARDWARE - Engine mounting bolts and similar hot zone hardware are plated with electroless nickel to in-crease useful life. Electroless nickel also provides uniformity of deposit on bolt threads-an important requirement.
DRILLS AND TAPS - Use of heat-treated electroless nickel plate extends wear life 20.30% over hard chrome plate for most applications of drills and taps.
UNDESIRABLE ASPECTS OF ELECTROLESS NICKEL
Electroless nickel plate, like hard chrome plate, will not take severe impact. There is a tendency to chip at sharp edges if impact forces are used. Since the melting point is 1635°F, high temperatures will cause failure. Heating to 1350°F will lower the hardness to near the as-plated condition.
Variables affecting hardness of electroless nickel
Electroless nickel plate is difficult to strip chemically. This is especially true after heat treatment. Freshly deposited electroless nickel may be stripped more easily.
Poor adhesion of electroless nickel plate has been reported for some
materials such as heat-treated 1280 steel used for
hydraulic housings. Also, some aluminum alloys have been re-ported to produce poor adhesion. However, it is the author's belief that with proper pre-plate preparation most aluminum alloys used in the aircraft industry can be electroless nickel plated with excellent adhesion.
OPERATION AND CONTROL OF ELECTROLESS NICKEL SOLUTIONS
Variables which affect deposit rate and hardness and quality of deposit are as follows:
1. pH value affects deposit rate and hardness. The lower the pH, the slower the deposit rate, and the greater the deposit hardness.
2. Sodium hypophosphite content affects deposit rate. It is important, therefore, to maintain sodium hypophosphite content by methodical replacement in the plating
bath based on mil/ft2 of plated surface. A coverage of 1 mil/ft2 requires approximately 4 oz of sodium hypophosphite. One pound of nickel deposited will consume approximately 5 lb of sodium hypophosphite at 200 F.
3. Bath temperature (190-210°F) affects deposit rate. At 190°F, the average bath will deposit at the rate of ap-proximately 0.5 mil/h; at 210°F, the average bath will de-posit at the rate of 1.25 mil/h.
4. Concentration of complexing agents and type of complexing agents determine, to a large extent, deposit rate. For example, citrate-containing solutions are usually slow plating.
5. Contaminants, such as zinc, cadmium, sulfur compounds, cyanides, arsenic, antimony, bismuth, lead, and tin, slow de-posit rate.
6. Certain materials can be added to the bath to increase phosphorus content of the deposit, thus increasing deposit hardness
A new and interesting field has been entered by study and development of electroless nickel using boron compounds as reducing agents. It is now possible to produce essentially pure nickel from an electroless system. Other interesting proper-ties are exhibited by these systems.
The advancing technology in the field of autocatalytic deposition is providing new and exciting finishes for surface treatment almost daily. Electroless nickel alloys are being developed to produce a variety of deposit properties such as extreme hardness, ductility, higher corrosion resistance, higher hot hardness properties, higher melting point, easily soldered nickel surfaces, and easily welded electroless nickel surfaces.
Wider usage by the aircraft industry of electroless nickel- plating has resulted from its unique properties of hardness, low surface friction, superior corrosion resistance, uniform deposit distribution, and high strength. New applications are being found as engineers better understand the properties of the deposit.
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