Electroless nickel (EN) plating is a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy. Due to its exceptional corrosion resistance and high hardness, the process finds wide application on items such as valves, pump parts etc., to enhance the life of components exposed to severe conditions of service ,particularly in the oil field and marine sector. With correct pretreatment sequence and accurate process control , good adhesion and excellent service performance can be obtained from EN deposited on a multitude of metallic and non-metallic substra6tes.
In the EN plating process, the driving force for the reduction of nickel metal ions and their deposition is supplied by a chemical reducing agent in solution. This driving potential is essentially constant at all points of the surface of the component, provided the agitation is sufficient to ensure a uniform concentration of metal ions and reducing agents. Electroless deposits are therefore very uniform in thickness all over the part's shape and size. This process offers distinct advantages when plating irregularly shaped objects, holes, recesses, internal surfaces, valves or threaded parts. Distinct advantages of EN plating are:
- Uniformity of the deposits, even on complex shapes.
- Deposits are often less porous and thus provide better barrier corrosion protection to steel substrates, much superior to that of electroplated nickel and hard chrome
- The deposits cause about 1/5th as much hydrogen absorption as electrolytic nickel and about 1/10th as much hard chrome.
- Deposits can be plated with zero or compressive stress.
- Deposits have inherent lubricity and non-galling characteristics, unlike electrolytic nickel.
- Deposits have good wetability for oils
- In general low phosphorus and especially electroless nickel boron are considered solderable. Mid and high phosphorus EN's are far worse for solderability
- Deposits are much harder with as-plated microhardness of 450 - 600 VPN which can be increased to 1000-1100 VHN by a suitable heat-treatment
The versatility of electroless nickel plating is demonstrated by the wide range of coatings possible. The following are important types of coatings industrially available (reference):
- Low Phosphorous (Hard)
A unique bath providing an as-plated deposit hardness of up to 60 Rockwell This bath provides a deposit nearly as hard as Hard Chrome, with the advantage of a uniform thickness inside complex configurations, as well as outside. The deposit is so uniform that grinding after plating is eliminated. Low Phosphorous Electroless Nickel offers excellent resistance to alkaline corrosive environments.
- Medium Phosphorous (Bright High Speed)
This is a workhorse electroless nickel. It has proven itself over the years. Steel parts plated with Medium Phosphorous electroless nickel will in many cases perform like stainless steel. Electroless nickel will not build up on edges or ends, and it plates inside and out giving uniform total coverage. With heat treatment, medium phosphorous electroless nickel can be hardened from 45 Rockwell C to as high as 68 Rockwell C.
- High Phosphorous
This finish provides maximum corrosion resistance. High Phosphorous electroless nickel is standard in industries that require resistance to strongly acidic corrosive environments like oil drilling and coal mining. High Phosphorous electroless nickel has a low degree of solderability. It will remain solderable for only a brief period of time after plating. This makes it a desirable finish for electronics parts such as connector housings and semiconductor packaging.
- Electroless Nickel/Teflon Composite
Teflon adds to the already slick surface of the electroless nickel, yielding a very low friction surface. This product is a relatively new one. It consists of microscopic beads of Teflon co-deposited up to 20% with the electroless nickel. This finish can be the solution to sticking, galling or drag problems with moving parts, or heated seal surfaces. In some cases, liquid lubricants can be eliminated with the use of Nickel/Teflon plating.
- Electroless Nickel on Zinc Die Cast
Electroless nickel can be applied directly to zinc die cast without a copper layer. This has many applications where corrosion resistance and resistance to chipping and flaking is necessary
The selection of a specific grade of EN-plating is done in accordance with the nature of application, where a high hardness and low coefficient of friction is desired, low phosphorous EN is preferred (1-3%P). For general applications where a bright finish is required and the operating conditions are not very corrosive, medium phosphorous (6-8% P) EN is used e.g.. Computer printer rollers, machine components, plastic molding dies etc. When the conditions of use for an EN plated components are severely corrosive, a high phosphorous EN (12-13% P) is usually selected. The high phosphorous EN is amorphous in nature and is compressively stressed unlike the low and medium phosphorous EN which are Crystalline and tensile stressed. Proper process sequence and maintaining the correct operating parameters helps ensure a virtually non porous deposit of high phosphorous ENP which finds wide application in areas such as valve components, aerospace industry, oil & gas and chemical industries etc.
- Surface Hardness: As plated 48-50 RC. After Heat Treatment (400°C, 1 hr) 62-63 RC
- Melting Point :- 890°C
- Density :- 7.85-7.95 gm/cm qb.
- Coefficient of friction :- EN Vs STEEL 0.3
- Coefficient of Thermal Expansion :- 0.13 microns /°c
Specifications and Testing
ASTM specifications are generally followed in evaluating EN plated components some of the relevant tests are as follows
- Hardness: The Hardness may be determined on a Microhardness Tester using a 100 gm load as per ASST. B-578
- Thickness: The microscopic examination of the cross section of an article is tested in accordance with ASTM B-478. The ENplating thickness will vary from 5-125 microns depending upon the service conditions.
- Corrosion Resistance: A 5% neutral salt spray test is carried out as per ASTM B-117 to determine the corrosion resistance of plated items. This is most important test in evaluating EN plated samples. The Corrosive conditions to which EN Plated components are exposed, can be classified as mild, moderate and severe. The bath used for EN Plating, varies accordingly to obtain alloy compositions varying from low to high phosphorous content. Generally a high corrosion resistance requires a high phosphorous content (10-12% ) and the relevant test to evaluate the performance of plated samples is the 5% Neutral salt spray test in accordance with ASTM B-117. With a proper operating procedure, high phosphorous deposits will show no rust spots, even after 1000 hrs of salt spray exposure for a plating thickness of 40-50 microns.
- Adhesion: Several tests such as Bend test, Quench test, Ring shear test etc. Are carried our to determine the adhesion of EN-plating to the base metal in accordance with ASTM-B-571.
Areas of Application
Due to its unique properties of excellent corrosion resistance, combined with a high wear resistance and uniformity of coating, EN finds extensive applications in a number of fields. Some of the prominent areas of application are:
- Oil & Gas: Valve components, such as Balls, Gates, Plugs etc. And other components such as pumps, pipe fittings, packers, barrels etc.
- Chemical Processing: Heat Exchangers, Filter Units, pump housing and impellers, mixing blades etc.
- Plastics: Molds and dies for injecting and low and blow molding of plastics components, extruders, machine parts rollers etc.
- Textile: Printing cylinders, machine parts, spinneret's, threaded guides etc.
- Automotive: Shock Absorbers, heat sinks, gears, cylinders, brake pistons etc.
- Aviation & Aerospace: Satellite and rocket components, rams pistons, valve components etc.
- Food & pharmaceutical: Capsule machinery dies, chocolates molds, food processing machinery components etc.
Solderability of E/N coatings
Most suppliers of E/N now recommend using low phos. for the best solderability, and longest shelf life. Standards ISO 4527, DIN 50966 and ONORM C 2550 (Austrian) reference this important property.
A paper submitted at the Electroless Nickel conference of 1989 held in Cincinnati Ohio, Titled "Solderability Parameters of Elecroless Nickel Bearing Electronic Finishes" By Louis Kosarek of STB Systems, Inc. report that "An electroless nickel deposit which contains a concentration of phosphorus ranging from 0.1% to 3.0% is readily solderable on an "As-plated Basis" per Mil-Std 883c method 2003. The frequency of solderability tests which fail per Mil-Std 883c will increase as the phosphorus content of electroless nickel alloy increases from 3.0 to 7.0% phosphorus. A solderability test conducted per Mil-Std 883c method 2003 incorporating an as-plated surface finish containing phosphorus in excess of 7%, the components will consistently fail. The mode of failure is non-wetting of the surface."
Mr. Kosarek's work is referenced in "Electroless Nickel Plating" by Wolfgang Riedel, Published by ASM International, ISBN 0-904477-12-6, under physical properties. This book is the best source for information on E/N plating, in my humble opinion.
On a personal note, most solderability problems I have heard over the years is when a purchaser switches vendors, and the new vendor does not have low phos. nickel, and uses mid. or high phos. instead or the vendor simply stops using the low phos. because of low demand and substitutes mid or high phos. thinking they are the same. I have heard of many solderability problems "disappear" when purchasers required certification to the low phos. requirement from their supplier.
The present consensus seems to be "the purer the nickel alloy the better the solderability."
Paul Szymanowski, Micro Plating, Inc.