The growth of electroless plating is traceable to many factors, but three key drivers over the years helped spur growth of the technology: 1)the discovery that some alloys produced by electroless deposition have unique properties compared with other electroplating technologies, notably nickel-phosphorus; 2) growth of the electronics industry, especially the development of printed circuits; and 3) large-scale introduction of plastics and other types of substrates benefiting from electroless coatings to meet many types of engineering requirements.
Electroless nickel (EN) plating systems became commercially available in the late 1950s and gained some acceptance throughout the ’60s, ’70s and ’80s. Their use has continued to expand since that time, but not without some growing pains in the form of misapplication and misunderstanding of their properties. Deposit failures impacted engineers’ perceptions of EN coatings and often gave the technology a bad reputation.
Of course, there were also many successes, but it took some time for many applicators, specifiers and suppliers of these technologies to help differentiate customer and chemistry operating parameters and understand relationships to the performance of deposits.
In the early years, chemical suppliers of EN technologies may have focused predominantly on positive attributes because any negative attributes would expose vulnerability and weakness to the market and their competition. Early on, many users of these deposits were primarily concerned with how the deposit looked on the part and how much the coating would cost. Sometimes applicators would be charged a bit less for a substandard deposit, which resulted in failure and created the false perception in the market that EN systems could not provide a consistent quality coating.
Throughout the 1990s the technology reached a maturity level, and those providing EN were better at educating the market with respect to the strengths and weaknesses of specific formulations. As a result, EN continued as a viable technology for many types of applications.
ELV and ROHS Initiatives
With the End of Life Vehicle (ELV) and Restriction of Hazardous Substances (RoHS) initiatives introduced after 2000, the EN technology that had existed since the 1960s required redevelopment to limit and eliminate use of lead stabilizers and cadmium brighteners. This current period of redevelopment now affords many chemical suppliers and industry educators an opportunity to improve the overall education process for EN technology, and hopefully, make up for past deficiencies.
EN specification documents—vital for suppliers, purchasers and consumers— are intended to help coating specifiers and applicators understand performance requirements and assist them in reaching agreement on expectations in supplying EN-plated parts. Specifications may be written by government agencies; standards organizations, such as the American Society For Testing and Materials (ASTM) and International Organization for Standardization (ISO); trade associations like the Materials Information Society (ASM International); private corporations; and many others.
Evolving ASTM Standards
A well-written specification provides good insight and performance but requires some level of maintenance. The evolution of ASTM documents is a good example of how specifications improve with time.
ASTM B 733-90, for example, had been approved in 1990 and has since been revised twice, in 1997 and 2004. The updated version is clearer and more concise than the earliest version and actually provides a good starting point for understanding EN technology.
Part drawings can include out-of-date specifications that can potentially result in problems. Most specifications do not change as dramatically as the ASTM B 733 did, but good communication between the purchaser of the coating and the applicator can help avoid a misapplication.
Resistance to Friction
Because Ni-P alloys possess a natural lubricity, all EN coatings have some level of resistance to many types of wear situations. Improved wear resistance allows softer parts or components that would typically have poor abrasion resistance to be used in applications otherwise not possible.
There have been studies of the relationships linking deposit phosphorus levels to the resulting deposit hardness both before and after post-plating heat-treatment processes. Depending on phosphorus content, EN deposits can be amorphous (>11 wt% P), crystalline (>4.5 wt% P) or a mixture of both (5–10 wt% P). Heat treatment of these deposits to different temperatures for given durations of time will cause structural changes, specifically crystallization through the formation of nickel phosphide (Ni3P), to occur in these deposits, which is responsible for hardness improvement.
Tribology—the study of wear—is relatively new compared with many other engineering topics, but over the years there have been many theories developed to describe mechanisms for various types of wear. These include corrosive or chemical, erosion, cavitation, fatigue, fretting, impact, sliding, abrasion and adhesive wear interactions.
In real-time wear interactions, metal-to-metal contact and the condition and hardness of the contacting surfaces must be taken into account. Wear testing can be complicated, both inside and outside of the laboratory, which is another reason Ni-P deposit hardness has traditionally been used as a gauge to indicate wear performance. It has been accepted that if the EN deposit is hard, or hardened by post-plating heat treatment, the deposit will have improved wear performance. Acceptance of this premise helps to perpetuate the deposit-hardness falsehood.
Defining the wear resistance requirement for a deposit through the type of testing is important, but we must also acknowledge that testing any deposit in a comparative environment under similar conditions only serves as a predictor of performance under that specific set of conditions.
Traditionally, abrasive wear, adhesive wear and sliding wear have been the most wear types most often documented for Ni-P alloys. Wear performance has also been shown to be primarily a function of the deposit hardness, and this has been related to alloy phosphorus content. ASTM B 733 references the Falex method (ASTM D 2670) of testing for adhesive wear, the Taber method (ASTM D 4060) for abrasive wear and the Alpha LFW-1 method (ASTM D 2714) for friction and wear testing.
Adhesive Wear Tests
Adhesive wear tests show relationships between interacting surfaces, but access to these tests and equipment can be costly. For this reason, adhesive wear testing often isn’t practical compared with abrasive wear tests, which are the type predominantly used to characterize coatings, including EN. Abrasive wear testing is more easily performed in your own lab because the equipment is readily available in the market.
Used to evaluate wear under conditions of dry abrasion, the Taber abraser test measures the weight loss of a rotating plated specimen panel by two dressed rubber-bonded abrasive wheels, usually using a 1,000-g load. Specimen wear is reported as the Taber wear index (TWI) in average weight loss in milligrams per 1,000 cycles. The Falex wear test has been used to measure adhesive wear with variables including load and revolutions per minute under both lubricated and non-lubricated conditions.
A study done in the late 1980s titled “Hardness & Wear Resistance of Electroless Nickel Alloys” is particularly relevant, because the same Ni-P alloy deposits were evaluated with two different test methods and their correlation to deposit hardness in the “as deposited” and “as heat treated” conditions were examined. Tables II, III and IV present a summary of that data.
In the abrasive wear result, the Taber wear example data shows that high deposit hardness does not always equate to the best wear resistance. However, when looking at a different wear mechanism of adhesive wear using the Falex test method, we see a different outcome with the same deposit.
The data in Table II clearly show that, for the as-deposited alloys, the 4.4-percent P deposit has both the highest average hardness at 700 HK100 and the lowest resulting TWI (9.1-mg loss) compared with both higher-phosphorus deposits. Table III shows that, as the 4.4-percent P deposit is heat treated at two temperatures (350 and 400°C), deposit hardness is lower than that of the 11.2-percent P alloy. However, TWI values show that the 4.4-percent P alloy has lost only 9.9 mg at both heat treatment temperatures, while the 11.2-percent P alloy has higher TWIs of 14.1 and 10.6 mg, indicating improved abrasive wear with a harder, heat-treated deposit surface.
Falex wear results shown in Table IV indicate several important characteristics. For example, the 11.2-percent P deposit on the pins had the highest average hardness (906–910 HK100), but had no better adhesive wear performance than the 9.1-percent P alloy. It was, however, much better than the 4.4-percent P alloy.
High Hardness, Low TWI
The resulting wear on each of the V-blocks (hardness Rc 20–24) from each plated pin shows the 9.1-percent P alloy had the least total wear impact on the V-block at both heat treatment temperatures. The other two deposit compositions had much more adhesive wear impact on the V-blocks.
Adhesive wear testing is challenging because of all the potential variables. Test parameters, including load, lubrication (if any), amount and type of lubrication, duration of test, break-in period, hardness of V-blocks, and coating on V-blocks, will likely change the outcome.
Many conclusions can be drawn from this study. However, it does show that the 4.4-percent P alloy had better Taber (abrasive wear) performance and poorer Falex (adhesive wear) performance, and that its range of average microhardness after heat-treatment was not the highest. Additionally, the high-phosphorus (11.2-percent P) deposit, despite having the highest average microhardness after heat treatment, had poor abrasive wear resistance and not the best adhesive wear result.
Specifying Details is Important
More significantly, the data show that the belief that the highest deposit microhardness provides the best wear performance does not always hold true. Specifying details and qualifying the test methodology to be used are very important.
Since the early days of EN development and commercialization, there has been much written about the coating’s ability to provide a level of defined corrosion performance in the environment where it’s exposed. EN deposits have shown this ability to be true, primarily as a result of their low porosity and resistance to chemical attack.
EN deposits, like electrodeposited nickel, are cathodic coatings over most substrates. As a result, these function as barrier coatings and act to protect substrates by a mechanism of encapsulation which helps to seal them off from the exposure environment. Once this barrier is penetrated, the protective value of the deposit is lost, and the substrate is subject to corrosion. In contrast, anodic coatings such as zinc plating over steel provide protection to the substrate by sacrificially corroding themselves relative to the substrate.
Recognizing that corrosion test methods are most useful for relative comparative purposes between deposits or in similar environments, which may or may not represent actual service conditions, is important. For this reason, the best approach is testing of EN deposits in the exact environment and under the conditions of exposure they will see in service. Of course, this approach is not always practical, resulting in the development of a number of alternative exposure tests, including neutral salt spray.
Any defined degree of corrosion performance for an EN deposit is determined by many factors related to its exposure. For example, will the deposit be exposed to an acid or alkaline environment? At what given concentration of media? Will there be an oxidizing or reducing atmosphere, at what exposure temperature? Is the exposure wet or dry? These factors for the resulting deposit make predicting corrosion performance of EN deposits very difficult. For this reason, actual time exposure tests to the environments where the EN deposits will be used are more meaningful.
Corrosion Tests to Use
But, because real time corrosion resistance is reflected by resistance to attack by chemical reaction, there are many corrosion tests that can be used to evaluate Ni-P deposits. For example, the resistance of deposits to blackening in nitric acid is a common test used mostly in the electronics industry. A high-phosphorus deposit (>10 wt% P) provides better resistance to nitric acid exposure (no blackening of the deposit) with minimal attack compared with lower-phosphorus EN deposits. High-phosphorus alloys also provide the best overall corrosion resistance in the widest variety of environments; however, their resistance has been shown to be relatively poor in strong alkaline mediums, and low-phosphorus EN deposits provide the best corrosion performance overall.
It’s important to note that all EN deposits provide some level of corrosion protection by the degree of encapsulation of the coated substrate. But, do all EN deposits have the same corrosion performance? The simple answer is no, because porosity in these deposits are primary points of failure in the coating. Of course, higher-phosphorus EN deposits have the potential for producing the least porosity, a result of the homogeneous-amorphous structure, so generally speaking these types of systems provide the best corrosion protection.
Effect of Porosity
It is accepted that high-phosphorus EN provides the greatest protection in the widest exposure situations because it has the lowest porosity and the highest deposit passivity compared with deposits with lower phosphorus contents. The high corrosion protective nature of high-phosphorus deposits is related to their amorphous structure as deposited. Still, all ranges of EN deposits provide some level of corrosion protection.
In various exposure environments, phosphorus content has been shown to have a significant effect on the coating’s protective value, which is also referenced in ASTM B 733, Appendix X5 with the Kure Beach corrosion study. The study showed that after many years of exposure to that environment, improved corrosion ratings (per ASTM B537 practice) of 9.5 or higher were obtained with higher phosphorus deposits. A rating of 9.5 represents 0.05 percent or less of the surface area showing corrosion.
Addressing this salt spray misconception, there are many references to EN deposits passing salt spray per ASTM B117, but the first problem is that there are no criteria of “required hours of performance” listed within that document to determine what constitutes a passing score or how much corrosion is allowed.
And what constitutes passing can be relative. First signs of substrate corrosion? Up to a given number of spots over a given surface area? Up to a given percent of the surface area exposed? All are important considerations that need to be defined before passing judgment on whether an EN deposit will pass or fail salt spray porosity testing.
Many industry experts have argued for years against use of salt-spray testing, specifically ASTM B 117, to evaluate the corrosion resistance of nickel coatings on steel, copper, aluminum or other materials where nickel is cathodic. A summary of more relevant concerns is found in Table V.
My own participation over the years in many salt spray studies with EN deposits has shown inconsistencies. Commonly used test results can be misleading regarding how an EN deposit will perform in corrosion tests. ASTM B 733-97 recognized this deficiency and eliminated salt-spray testing even as a porosity test in later revisions.
Ultimately, it was recognized that neutral salt fog spray was not very corrosive to nickel, so there was no advantage in specifying the test. If specified, it should be agreed upon that in the best case, a salt-spray test is better suited as a porosity test. The argument then becomes that there are much simpler, faster tests that exist, and many of these choices are outlined in ASTM B 733-04.
Unfortunately, many OEM specs and other proprietary specs continue to abide by the belief that salt spray testing EN deposits is a good corrosion resistance test.
That aside, the salt spray performance of EN is a direct function of the resulting deposit porosity. While many have studied the factors that influence porosity, it has been shown that increasing nickel thickness improves the corrosion protection of the deposit. For a given thickness, however, the degree of corrosion protection to the substrate is influenced by many variables ranging from the incoming surface condition of the substrate to the post plating practices such heat treatment for improving hardness or hydrogen embrittlement relief. Every process step in between for depositing EN on a part also plays a critical factor regarding porosity. It is not only the operation of the EN that contributes to potential failure.
Because the presence or degree of porosity in the EN deposit will affect corrosion performance, ASTM B 733 provides a good source of information regarding minimum coating thickness requirements for EN deposits in various service conditions. Table VI correlates Ni-P alloy thickness requirements with their intended applications. However, this does not take into account any potential impact of the substrate condition on meeting the final result.
High-Phosphorus Alloys Less Porous
Experience has shown that since higher phosphorus alloys have less porosity overall and where thinner deposits are required, a > 10-percent P alloy will tend to out-perform a 7-percent P alloy which also in turn performs better than a 4-percent P alloy. In all cases, increasing the deposit thickness is shown to improve the neutral salt spray performance relative to porosity, as shown with a study of the impact of surface finish versus EN type in Table VII.
Since most of the commercialism of EN systems began in the 1960s, finishing shops using the technology installed various size tanks to plate EN finishes on parts of many sizes and configurations. These pioneers experienced the reality of solution life limitations.
The conventional EN chemistry still defined as standard today is a 6 g/L nickel solution that uses from 25 g/L or higher sodium hypophosphite as the reducing agent resulting in the formation of the Ni-P alloy deposit.
There are many types of electroless nickel baths, however acid nickel sulfate/sodium hypophosphite baths are commonly recognized as standard or conventional. One commonly accepted mechanism for the reduction and deposition of nickel in a hypophosphite reduced bath is as follows:
NiSO4 + H2O → Ni++ + SO4 = + H2O (1)
NaH2PO2 + H2O → Na+ + H2PO2- + H2O (2)
Ni++ + H2PO2- + H2O → Nio + H2PO3- + 2H+ (3)
H2PO2- + H2O H2PO3- + H2↑ (4)
Equations (1) and (2) simply show the dissociation of the nickel salt and sodium hypophosphite in water. Sulfate and sodium are the by-products of these two reactions that build up with usage. Equation (3) shows the reduction of the nickel ion by hypophosphite to form nickel metal on the parts and an orthophosphite anion which is another major by-product. Equation (4) shows a parallel reaction of hypophosphite with water to form orthophosphite and hydrogen gas. The hypophosphite to metal ratio varies with proprietary systems and is consumed in a given ratio to the nickel metal concentration. Typically for high phosphorus (10–13 percent) baths, the efficiency is 5.6 g of hypo consumed for every 1 g of nickel deposited. For medium phosphorus (7–9 percent) baths, 5 g of hypo are consumed for every 1 g of nickel. For the low-medium phosphorus (4–6 percent) systems, 4.2 g of hypo are consumed for every 1 g of nickel deposited.
From the by-products produced perspective for every 1 g of nickel metal plated, 3.9 g of orthophosphite ion (5.0 g as sodium orthophosphite), 1.64 g of sulfate, and 1.1 g of sodium are produced, and remain in the plating bath as the solution ages.
A Self-limiting Process
The build-up of these major by-products over the course of the bath life is the primary reason EN is a self-limiting process. This build-up, specifically of sodium orthophosphite, but other constituents as well, increases the density of the solution at a rapid rate that eventually causes a reduction in the solubility of other components. When the total dissolved solids as measured by specific gravity reaches 1.28 g/mL or higher, deposit problems and solution problems are the result.
The increase of sodium orthophosphite also reduces the deposition rate of the bath. As these orthophosphite levels increase, the deposit smoothness, brightness, plating rate and adhesion can be negatively affected, resulting in roughness, pitting or porosity of the deposit. Typical sodium orthophosphite levels at the end of a bath life vary depending upon many factors. Most EN systems being replenished with 30 g/L sodium hypophosphite per metal turnover produce 35–36 g/L of anhydrous sodium orthophosphite. For every 1 g of sodium hypophosphite consumed, 1.2 g of anhydrous sodium orthophosphite will be produced. The steady increase of orthophosphite content can easily be monitored by measuring the specific gravity of the solution at a constant temperature or by titration analysis. Solution loss has a possitive impact toward extending the EN solution life.
This finite solution life requires the EN plating solution be discarded after plating as little as 36–48 g/L of nickel metal unless using some type of life extension strategy. This is confirmed in the lab, where operating a 1 L bath of EN chemistry under a controlled protocol exposes the myth of being able to obtain more than eight metal turnovers.
It’s always important to qualify the meaning of MTO with each type of chemistry being used. Here, for a standard type nickel chemistry, a metal turnover (MTO) represents every time 6 g/L of nickel is replenished back to the bath. (Today, with low metal chemistries, a MTO can be much different when a 3 g/L chemistry can easily achieve a life of 16 MTOs, plating 48 g/L of nickel, which on the surface appears to be extraordinary.)
Unless these by-products are removed either through solution drag-out, bleed and feed methods or by using membrane or electrodialysis technology to extend the life, there is no turning back on making up a new standard chemistry solution at about every eight MTOs, or possibly 10–12, given the right circumstances.
But without life extension chemistry or equipment, there’s little hope to achieve 30 MTOs or more. There are long life chemistries available in the market that use alternative sources of nickel metal salts including nickel-hypophosphite and nickel acetate that can achieve higher solution life. Of course, every option needs to be weighed cost versus performance.
High Amorphous is Preferred
It’s already been shown that a higher degree of amorphous character in the EN deposit is preferred for optimum corrosion resistance performance. So any material added to an EN system—either by design or as a contaminant—that alters the EN microstructure by creating a less amorphous character will impact corrosion performance.
For example, certain functionalized organic additives, such as thiourea and other sulfur additive types, increased the crystalline character of the deposit. This in turn results in increasing deposit porosity and thus a reduction in corrosion protection. Only a very few organic additives result in a more amorphous deposit structure, but in general, adding sulfur-bearing organic additives to a high-phosphorus EN is detrimental.
Surface active agents (SAAs) are another class of functionalized organic molecule that can improve the properties of EN films and assist with plating of very thick, pit-free EN deposits (>25µm) SAAs function by reducing the interfacial surface tension between the catalytic surface and the EN solution—increasing the wettability of the substrate.
Lower surface tension minimizes adsorption of particulates, hydrogen gas, and colloidal impurities, reducing the amount of micro-defects in the deposit. To perform effectively, the SAA must be compatible with the EN solution chemistry in that it must not separate from the solution at operating temperature, foam excessively, or break down on prolonged heating.
Small amounts of SAAs can reduce micro-pitting and improve the corrosion resistance of an EN deposit. The most common types of SAAs used in EN formulations are non-ionic or anionic; cationic surfactant types are generally avoided because they are too strongly adsorbed to the plating surface and result in pitting or poor adhesion of EN deposits.
One study of the effect surfactants have on properties of a high-phosphorous nickel formulation (10 percent P) evaluated nine different surfactants with varying structures and charges. It was found that, at very low concentration ranges from 3–7 mg/L, the deposition rate of the EN solution could be increased by 25 percent compared with the surfactant free solution.
Reduction In Micropitting
The same study also reported a significant reduction in deposit micro-pitting while the corrosion resistance was enhanced, particularly when the resulting deposit was exposed to an acidic environment. A low concentration (5 mg/L) of a polyoxyethylene sorbitol ester non-ionic surfactant in a plating bath resulted in 60 percent less deposit weight loss compared with the bath with no surfactant after exposure to 10 wt% hydrochloric acid for 16 days.
Understanding surfactant impact on deposit microstructure and corrosion rates, especially for high-phosphorous EN films, is an important formulation objective for chemistry suppliers. A primary goal is to produce smooth, uniform, defect-free deposits with the smallest possible number of phase boundaries in the microstructure, which corresponds to improved corrosion resistance.
Four different types of SAAs investigated in an internal study at Coventya have been found to significantly reduce micro-pits in the resulting deposits. The study also evaluated both high and low solution agitation because of the potential impact of solution dynamics on EN corrosion performance. Table VII shows the SAAs evaluated, their charge and their relative surface tension reduction.
EN deposits plated from solutions containing these SAAs underwent corrosion testing by exposure of test coupons to 50 wt% nitric acid, and a measure of their weight loss was observed. Addition of the lignosulfonate SAA resulted in the lowest weight loss at low solution agitation but also the highest weight loss with high solution agitation. From a formulation standpoint, this would be unacceptable, because solution movement variability exists in most EN installations. The hydrocarbon anionic SAA type is shown to have the best overall nitric acid corrosion performance, and would be chosen to improve deposit microstructure without negatively impacting acid corrosion resistance.
Carefully selecting the proper organic additive can offer benefits to EN deposits. Limiting exposure of the EN plating solution to organic contaminants is also critical to assure the maximum corrosion performance from these deposits for many applications.