Electroless Nickel Myths Busted

Article From: Products Finishing, , from Coventya Inc.

Posted on: 11/1/2008

The word “myth” is derived from the Greek word mythos, which means “word of mouth” in many dictionaries.

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Taber abraser

Types of wear tests available include the Taber abraser, for abrasive wear, and the Falex test for adhesive wear. The Taber test is much more commonly used for ENP deposits and other coatings.

Effect of various SAAs

Effect of various SAAs on corrosion rate of ENP deposits in 50% nitric acid. Performance of some SAAs in affecting deposit microstructure and inhibiting corrosion was highly dependent on the degree of solution agitation.

Various SAAs can lower surface tension

Various SAAs can lower surface tension and thus improve wettability of the substrate being plated to reduce adsorption of particulates, hydrogen gas and colloidal impurities by the deposit. A contact angle measurement of less than 90° indicates a reduction in surface tension.

The word “myth” is derived from the Greek word mythos, which means “word of mouth” in many dictionaries. In popular use, its meaning can also be a collectively held belief that has no basis in fact. The term is frequently associated with legend, fiction, fairy tale, folklore, fable, confusing data, personal desire and even urban legend.

In the realm of surface finishing, confusing data seems to be the best fit for a definition. There are many myths that have been associated with electroless nickel, specifically electroless nickel-phosphorus (ENP) alloy technology, over the years, and it’s surprising how much misinformation and misperception still exist with a technology that’s been commercially available since the late 1950s.

With the desire to be a “myth buster,” coupled with 26 years of experience in surface finishing—a good portion of this focused on ENP technology, working with R&D personnel, applicators, engineers and others who specify ENP deposits—In this article, I will try to debunk some of the myths surrounding ENP technology.

Why Electroless Nickel?

The growth of electroless plating is traceable to many factors, but three key drivers over the years helped support the growth and utilization of the technology.

These are:

  • The discovery that some alloys produced by electroless deposition, notably nickel-phosphorus, have unique properties compared with other electroplating technologies.
  • Growth of the electronics industry, especially the development of printed circuits.
  • Large-scale introduction of plastics and other types of substrates benefiting from electroless coatings to meet many types of engineering requirements.
Table I—Significant Properties of ENP deposits
Deposit appearance
Alloy composition
Adhesion to substrate Corrosion resistance
Porosity—corrosion protection Wear resistance
Uniformity of substrate coverage Microhardness (hardness) of deposit
Deposit thickness capability Fatigue strength & deposit elongation
Electrical resistivity Internal stress
Solderability and melting range Magnetic tendency

Ultimately, the suitability of any type of electroless process for a specific application has to be property driven. For ENP systems, the resulting deposit properties, as shown in Table 1, are related to the amount of phosphorus co-deposited with the nickel. Whether one likes it or not, changing phosphorus in the alloy changes the resultant property.

While selecting the ENP process that produces the optimum phosphorus content for a given application can sometimes be a difficult task, it’s the lack of adequate understanding of the relationship and impact of phosphorus content and key deposit properties that has caused many myths to develop.

Perpetuating Myths

ENP plating systems became commercially available in the late 1950s and gained some acceptance throughout the 1960s, ’70s and ’80s. Their use has continued to expand since that time, but not without some growing pains in the form of misapplication and, at times, significant misunderstanding of their properties, which evolved into myths about the technology. Deposit failures impacted engineers’ perceptions of ENP coatings and, often, gave the technology a bad reputation.

Of course, there were also many ENP successes, but it took some time for many applicators, coatings 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 ENP technologies may have focused predominantly on positive attributes during the education process because any negative attributes would expose vulnerability and weakness to the market and their competition. Early on, too often, 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 ENP technologies were better at educating the market with respect to the strengths and weaknesses of specific formulations. As a result, ENP continued as a viable technology for many types of applications. With the End of Life Vehicle (ELV) and Restriction of Hazardous Substances (RoHS) initiatives introduced after year 2000, the ENP technology that had existed since the 1960s required re-development to limit and eliminate the utilization 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 ENP technology and, it is hoped, make up for past deficiencies. This article will play a part by examining some more common ENP myths.

MYTH: If I follow an industry specification, my deposit will always be good.

MYTH BUSTED: Best success in ENP application is achieved by using an updated specification document.

The good news with this myth is that it projects some recognition that specifications are a good source of reference—vital for suppliers, purchasers and consumers of the materials, products or services they represent. 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.

Also important is knowing that ENP specification documents are intended to help coating specifiers and applicators understand performance requirements and them in reaching agreement upon specific requirements or expectations in supplying ENP plated parts or components. The weakness expressed with this myth is the implication that if the deposit can be linked to a specification then the deposit will be automatically good, which may or may not be true without some further qualification.

A well-written specification provides good insight and performance, but like most things requires some level of maintenance. The evolution of ASTM documents provides a good example of how specifications improve with time. It’s my belief that even well-written specification documents should evolve over time and continue to be updated to reflect new knowledge and experience. There continue to be misapplications of ENP technology that result from not staying current with recent versions of specification documents.

A few months ago, our company received a call from a good applicator of ENP technology asking whether the product our company supplied to them met specification ASTM B 733-90 for their new job in a nuclear industry application.

First, some background is needed. I have been a member of ASTM B 08 Committee and served on various subcommittees since 1990 and have participated in the improvement of many specification documents including the one in question. With all ASTM documents, the revision date follows the document number; in this case, the “90” references the year of approval.

In answering the caller’s question, it might have been easy just to say certainly, the deposit met the specification requirements. Of course, someone not involved with ASTM might not be fully aware of the possible implications of using an outdated specification. It is now 2008, and there have been revisions of this document—in this case, dramatic revisions, which needed to be reviewed to responsibly answer this question from the customer.

The ASTM B 733-90 standard has evolved now through two updated versions (1997 and 2004) and appears much different from that earlier version. Especially to someone newly introduced to the industry or someone wanting to specify ENP deposits, the updated version is clearer, more concise and actually provides a good starting point for understanding the technology.

The classification for the 1990 version included:

  • Service condition requirements based on deposit thickness
  • Types of coatings defined based on test requirements
  • Classes of deposits were based on post-plate heat treatment to increase coating adhesion or deposit hardness
  • Classification number scheme that identifies basis metal, alloy coating composition and thickness.
Table II—Abrasive wear versus ENP alloy composition
 
Phosphorus in alloy, wt%

4.4$
9.1%
11.2%
Average microhardness (Knoop)
as deposited
700
519
525
Average TWI as deposited (mg loss/1000 cycles, CS-10
wheels, 1000-g load)
9.1
19.5
24.3

The ENP deposits covered in this 1990 document were referenced to the tests required to be performed on the deposit. A Type 1 deposit required testing for appearance, thickness, adhesion and porosity, while Type 2 deposits had all the requirements of Type 1 with an additional hardness test. Type 3 deposits had the same requirements as Type 1 but also included alloy composition and corrosion test requirements.

 

Table III —Abrasive wear versus ENP alloy composition and heat treatment
Phosphorus in alloy, wt%

4.4%
9.1%
11.2%
Average microhardness (Knoop) at heat treatment temperature indicated
350°C
400°C
350°C
400°C
350°C
400°C

832

887

853

902

906

910

Average TWI after heat treatment (mg loss/1000 cycles, CS-10 wheels, 1000-g load)
9.9
9.9
10.3
11.5
14.1
10.6

The 1997 version of the standard expanded its scope to include more deposit background and information. For classification, it recognized that the amount of phosphorus co-deposited with the nickel was critical and impacted many of the performance characteristics which in earlier versions had been vague. This revision provided a critical and needed overhaul. Classification was simplified to three sections, which were prioritized differently.

Table IV—Falex adhesive wear versus ENP alloy composition and heat treatment
 
Alloy composition and heat treatment temperature


4.4% P
9.1% P
11.2% P
Average Knoop microhardness
350°C
400°C
350°C
400°C
350°C
400°C
832
887
853
902
906
910
Falex wear (mg loss)
on EN-plated pins, 200-lb load
0.3
1.1
0.1
0.1
0.1
0.0
Falex wear (mg loss) on EN-plated pins, 400-lb load
3.1
4.0
0.2
0.4
0.2
0.6
Total pins falex wear
3.4
5.1
0.3
0.5
0.3
0.6
Falex wear (mg loss) on steel V-blocks, 200-lb load
1.8
1.8
0.0
0.1
1.1
0.4
Falex Wear (mg loss)
on steel V-blocks, 400-lb load
-0.2
0.1
0.1
0.3
0.0
0.3
Total V-block Falex wear
1.6
1.9
0.1
0.4
1.1
0.7

ASTM B 733-2004 provides one significant change from the 1997 version: it provides a requirement for knowing the substrate tensile strength. One reason for this change has to do with increased emphasis on pre- and post-plating baking of various substrates for reducing the risk for hydrogen embrittlement. The evolution of this ASTM-B733 document reached a new high point with the 2004 version and today is much better for anyone involved with ENP coatings.

Getting to the root of the original customer question, the last updated part drawing (produced in 1991) called out the ASTM B 733-90 reference. Clarifying this situation determined what the application really required so the proper ordering information from the specification was obtained.

This example illustrates a common situation where part drawings can include an older specification version reference which does not get updated and can potentially result in a problem. True, most specification documents do not change as dramatically as the ASTM B 733 did in this case, but this exercise underlines the importance of good communication between the purchaser of the coating and the applicator, thus avoiding a misapplication and potentially a new myth about ENP technology.

Resistance to friction and wear is often cited as one of the features and a primary reason to utilize ENP deposits for many applications. And, because Ni-P alloys possess a natural lubricity, all ENP 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 utilized in many applications otherwise not possible.

MYTH: The hardest En deposit will always provide the best wear performance.

MYTH BUSTED: Not all hardened ENP deposits provide the best wear performance.

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, ENP 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.

Wear can also be thought of as the progressive loss or displacement of material from a surface as a result of some relative motion between that surface and another or the result of some action on that surface that causes displacement of material. The end result of poor wear resistance is normally recognizable in some failure of a component or system.

Tribology—the study of wear—is relatively new compared to 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, considerations regarding 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 gage to indicate wear performance. It has been accepted that if the ENP deposit is hard, or hardened by post-plating heat treatment, the deposit will have improved wear performance. Acceptance of this premise also helps to perpetuate the deposit hardness myth.

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 test methods show relationships between interacting surfaces, but access to these tests and their equipment can be costly. For this reason adhesive wear testing is often not deemed practical compared to conducting abrasive wear tests, which are the type predominantly used to characterize most coatings, including ENP. Abrasive wear testing is less complicated and more easily performed in your own laboratory as a result of the equipment being readily available in the market.

Table V—Minimum thickness requirements for Ni-P alloy deposits for
ferrous substrates
Minimum deposit
thickness, µm
Application examples
0.1 Diffusion barrier, undercoat, electrical conductivity, wear
and minimal corrosion protection in specialized environments
5 Light load lubricated wear, indoor corrosion protection,
soldering and mild abrasive wear
13 Industrial atmospheric exposure on steel substrates in dry or oiled environments
25 Non-marine outdoor exposure, or exposure to alkali salts and moderate wear
75 Exposure to acid solutions, elevated temperatures and
pressures, hydrogen sulfide and carbon dioxide, oil, high-
temperature chloride systems, very severe wear and marine immersion

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 under a 1000-g load. Specimen wear is reported as the Taber wear index (TWI) in average weight loss in milligrams per 1000 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 comprehensive study titled “Hardness & Wear Resistance of Electroless Nickel Alloys” was carried out in the late 1980s. The work 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 in the matrix. 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 methodology, 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% P deposit has both the highest average hardness at 700 and the lowest resulting TWI (9.1-mg loss) compared to both higher-phosphorus deposits. Table III shows that, as the 4.4% P deposit is heat treated at two temperatures (350°C and 400°C), deposit hardness is lower than that of the 11.2%P alloy. However, TWI values show that the 4.4% P alloy only has lost 9.9 and 9.9 mg at both heat treatment temperatures, while the 11.2%P alloy has higher TWIs of 14.1 and 10.6 mg, indicating improved abrasive wear with a harder, heat treated deposit surface.

Table VI—Experience with ENP in salt-spray (fog) testing
Inconsistent results are common. The same EN deposit, same thickness, on the same type of test panel can give results that vary from 24–1,000 hr to first substrate corrosion.

Mechanical surface finishing (dry grit blasting, aluminum oxide, or vapor blasting) of the substrate tends to give poor salt fog protection, even with thicker EN coatings.

Test panels with the same visible finish but from different vendors can give significantly different results, so how will actual parts with varying substrate conditions have good performance? Smoother test panel substrate finishes (bright polished) give less variation and improved corrosion protection at a given deposit thickness.

Over-pickling of the steel substrate decreases salt fog protection, as expected. Some steels are over-pickled very easily during processing.

Some ENP operating parameters—for example, tank loading and high solution agitation—tend to create deposits with greater porosity, which fail salt spray tests. Coatings deposited at a slower rate tend to be less porous. Higher-phosphorous EN alloys tend to be less porous (and also plate at a slower rate than mid-phosphorous EN).

Falex wear results shown in Table IV indicate several important characteristics. For example, the 11.2% P deposit on the pins had the highest average hardness (906–910), but had no better adhesive wear performance than the 9.1%P alloy. It was, however, much better than the 4.4%P alloy.

The resulting wear on each of the V-blocks (Hardness Rc 20–24) from each plated pin shows the 9.1% 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 particular 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, coating on V-blocks and other variables will likely change the outcome.

There are many conclusions to be drawn from this study; however, it does show that the 4.4% 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% P) deposit, despite having the highest average microhardness after heat-treatment, had poor abrasive wear resistance and not the best adhesive wear result.

More significantly, the data show that the myth—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 ENP development and commercialization, there has been much written and presented in the literature about its ability to provide a level of defined corrosion performance in the environment where it’s exposed. ENP deposits are known to provide some level of corrosion performance, primarily as a result of their low porosity (related to corrosion protection value) and resistance to chemical attack (related to corrosion resistance).

MYTH: A good salt spray test result tells me the deposit has very good corrosion resistance.

MYTH BUSTED: Commonly used salt-spray test results can be misleading.

Any defined degree of corrosion performance for ENP deposits is determined by many factors related to the environment of exposure. For example, will the deposit be exposed to an acid or alkaline environment? At what given concentration of media? In an oxidizing or reducing atmosphere, at what exposure temperature? Is the exposure wet or dry? These factors and others make predicting corrosion performance of ENP deposits very difficult. For this reason actual time exposure tests to the environments where the ENP will be used are more meaningful but not always practical.

The corrosion performance of ENP deposits is also proportional to deposit thickness and the corrosion test method utilized for evaluation. 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 ENP deposits in the exact environment and under the conditions of exposure they will see in service. As stated above, this approach is not always practical for a number of reasons, resulting in development of a number of alternative exposure tests.

Table VII—Types of SAAs evaluated in internal nitric acid exposure tests
Agent
Charge
Surface tension reduction
Lignosulfonate (LSDA)
Anionic
No
Nonylphenol (NPPA)
Non-ionic
No
Fluorocarbon
Non-ionic
Slight
Hydrocarbon (LIST)
Anionic
Significant

ENP deposits, like electrodeposited nickel deposits, are cathodic coatings (the cathode) over most substrates. By definition, 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.

Table VIII
Surface Active Agent
Property
Surface Tension
Reduction
Charge
LSDA
Lignosulfonate
No
Anionic
NPPA
Nonylphenol
No
Non-ionic
FCS
Fluorocarbon
Slight
Non-ionic
LIST
Hydrocarbon
Significant
Anionic

Effect of Porosity. It is accepted that high-phosphorus EN deposits have lower porosity than deposits with lower amounts of phosphorus. Still, all ranges of ENP deposits provide some level of corrosion protection to substrates. 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. High-phosphorus EN provides the greatest protection in the widest exposure situations because it has the lowest porosity and the highest deposit passivity compared to deposits with lower phosphorus contents. The overall high corrosion protective nature of high-phosphorus deposits is related to their amorphous structure as deposited.

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 several variables, including deposit chemistry and the roughness and porosity of the substrate itself—what I have in past publications referred to as getting the S.C.R.A.P. (Substrate, Cleaning, Rinsing, Activation and Plating) value from your ENP. Because the presence or degree of porosity in the ENP 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 5 correlates Ni-P alloy thickness requirements with their intended applications.

Knowing that all ENP deposits provide some level of corrosion protection by the success or degree of encapsulation of the coated substrate serves as good background. But, do all ENP deposits have the same corrosion performance? Of course the higher-phosphorus ENP deposits have the potential for producing the least porosity, a result of the homogeneous-amorphous structure, so generally speaking they provide the best corrosion protection of all ENP types.

But, because corrosion resistance is reflected by resistance to attack by chemical reaction, there are many corrosion tests that can be utilized 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) compared to lower-phosphorus ENP 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 ENP deposits provide overall the best corrosion performance.

Salt-Spray Testing. Many 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 VI.

My own participation over the years in many salt spray studies with ENP deposits has shown inconsistencies. Commonly used test results can be misleading regarding how an ENP deposit will perform in corrosion tests. ASTM B 733-97 recognized this deficiency and eliminated salt-spray testing even as a porosity test.

Ultimately, it was recognized that neutral salt fog 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 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 myth that “salt spray testing ENP deposits is a good corrosion resistance test.”

MYTH: Organic additives are bad for the corrosion performance of my deposit.

MYTH BUSTED: Organic additives of specific types can result in improved corrosion performance.

It has already been shown that a higher degree of amorphous character in the ENP deposit is preferred for optimum corrosion resistance performance. It should therefore be recognized that any material added to an ENP system —either by design or as a contaminant—that alters the ENP microstructure by creating a less amorphous character will impact corrosion performance in some manner.

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 potential. 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 ENP is recognized to be detrimental.

Surface active agents (SAAs) are another class of functionalized organic molecule that can improve the properties of ENP films and assist with plating of very thick, pit-free ENP deposits (>25 μm). SAAs function primarily by increasing the wettability of the substrate, specifically by reducing the interfacial surface tension between the catalytic surface and the EN solution.

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, must not foam excessively, and must not break down on prolonged heating.

Additions of small amounts of SAAs can reduce micro-pitting but also can improve the corrosion resistance of an ENP 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 ENP deposits. One study of the effect surfactants have on properties of a high-phosphorous nickel formulation (10% 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% compared to the surfactant free solution.

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% less deposit weight loss compared to the bath with no surfactant after exposure to 10 wt% hydrochloric acid for 16 days.

Understanding the impact that surfactants have on deposit microstructure and on corrosion rates, especially for high-phosphorous EN films, is a very important formulation objective for chemistry suppliers. A primary development 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 our company 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 ENP corrosion performance. Table 7 shows the SAAs evaluated, their charge and their relative surface tension reduction.

ENP 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 not be acceptable, because solution movement variability exists in most ENP installations. The hydrocarbon anionic SAA type is shown to have the best overall nitric acid corrosion performance, and would be chosen based on this evaluation to improve deposit microstructure without negatively impacting acid corrosion resistance.

Carefully selecting the proper organic additive can offer benefits to ENP deposits. Limiting exposure of the ENP plating solution to organic contaminants is also critical to assure the maximum corrosion performance from these deposits for many applications.

 



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