SCRAP is a concept that simplifies the important steps involved in preparing and plating surfaces with electroless nickel. These two case histories will shed some light on how the concept can be applied...
At Electroless Nickel ’97, I (Brad Durkin) presented the paper “Who Really Cares About the Surface Preparation Required for the Successful Application of Electroless Nickel?” That paper introduced the acronym SCRAP, which is a concept that simplifies the important steps in the plating process. The discussion included information about the Substrate, Cleaning, Rinsing, Activation and Plating steps for electroless nickel, each of which is required for a successful application. In other words, the quality of the electroless nickel deposit begins with the substrate and ends with the plating.
Too often the focus of solving plating problems only occurs at the plating tank, while the root cause of the problem frequently is hidden (or maybe lost) somewhere in the preparation process. One should not ignore the importance of the other elements of the SCRAP process. The ’97 paper provided a good overview of preparation practices, emphasized the important aspects of SCRAP and related each step to the production of a quality electroless nickel deposit.
Additionally, since Star Wars mania is back again with a fury, it was decided that a sequel to the SCRAP paper could be justified today. It will be beneficial to review the concepts of that presentation for a better background understanding of the subject discussed here. In this sequel, the primary focus will be on two case histories where the importance of SCRAP was fully realized.
A long-standing customer was using a high-phosphorus chemistry and had done so successfully for many years. This customer decided it was important to expand its product line. So, it moved forward to install another electroless nickel tank in the existing line. The customer chose a chemistry that plated a 5-6% w/w nickel phosphorus deposit because it complemented the high-phosphorus deposits it already provided. It already had the existing cleaning portion of the line (SCRA) and just needed to add the electroless nickel tank. Soon it would be ready to provide this new coating and take advantage of its characteristics.
Since there were production throughput limitations from the slower plating rates of the high-phosphorus chemistry, the customer wanted to transition the work from its largest customer to the lower phosphorus chemistry. Because the plating rate of the lower phosphorus system was almost twice the rate of the high-phosphorus system, it could produce more pieces per hour. (Additionally, the customer could discontinue the high-temperature (400C) hardness heat treatment since the new as-plated deposit had a hardness of 750 HK100.) This would meet its customers’ hardness requirements without the additional inert atmosphere heat treatment process. It soon became evident this changeover would be a win-win situation for everyone.
The day came when parts were going to be processed down the line using the new electroless nickel chemistry. No one expected a problem, nor did anyone expect this job to become an issue. After all, the company had successfully processed millions of these parts for many years. True, the new chemistry might need to be adjusted to rebalance the equilibrium of the line, but that was a normal situation to expect in all cases. What could go wrong?
The existing part being plated on the line was an induction-hardened high-carbon-steel automotive component. The tarnished appearance of the as-received raw part showed the signs of heat treatment. This had always been the case, and the customer’s existing preparation cycle had worked well to process these parts. The existing line consisted of the following cycle steps that, even today, is considered to be a good approach for these types of substrates.
The first load of parts was processed in the same manner as all previous loads. They were taken from the new electroless nickel solution and appeared flawless, like the shine on a new diamond in the showcase of a jeweler. The new electroless nickel chemistry had plated as expected: 0.75 mil/hr with a very uniform, semi-bright finish. The deposit appeared good, and the plating performance of the electroless nickel solution was predictable.
For the moment of truth, an adhesion test for the deposit was going to be checked. The test consisted of hitting parts together at random angles of deflection without any loss or separation of the electroless nickel from the substrate. There are many adhesion tests outlined in ASTM B571, but none of those could be used directly to characterize the deposit1. Let’s say this test was a modified impact test. The customer version was not a high-tech test, but one that characterized the condition of the coating’s adhesion with good reproducibility. Bang, bang, clink, clink. The areas of impact were examined. The results were as expected—good adhesion with no signs of flaking or adhesion loss. Everyone was happy, and each celebrated the victory in his own manner.
Production continued for several days. Suddenly, that ominous call was received on the third day. “Hey, your new nickel is coming off these parts. You better get over here.” The initial focus (or blame) was placed on the electroless nickel solution since it was the only thing that had been changed in the line! Yes, that was true, but other types of parts were processed through the new solution with good success. The chemistry was not smart enough to flake on one type of part while leaving the other alone, especially at the same nickel thickness. It was shown that parts were flaking after 2-3 days. Adhesion off the line was still excellent. Still, the electroless nickel chemistry was checked to verify there were no problems with the solution. A new solution was even made up, and parts were processed again. But, after 2 days, the coating began to flake again.
Our SCRAP sense told us that other areas also needed to be investigated. We attempted a post-plating bake (as opposed to heat treatment) at 250F that did not improve the adhesion on these flaked parts. Of course all the cleaners and acids were dumped and remade on the line, but the problem still existed. After many hours of laboratory and plant tests, long days of scratching one’s head, endless brainstorming sessions around the vending machines and many sleepless nights, the problem was fixed! We went back to using the high-phosphorus chemistry again, and the problem went away. Shrug! Both the as-plated adhesion and adhesion after 2 days were good again. Now, production could continue, but the challenge still remained to identify the root cause of the problem. There were many more questions than answers. How could this problem occur? What happened to cause this poor adhesion? Was the new 5-6% w/w phosphorus electroless nickel solution the problem? Other similar chemistries, including a competitive chemistry, were used with the same result—poor adhesion after a couple of days.
After many scanning electron micrographs and EDAX (energy dispersive xray) analyses of these defective surfaces, pilot line tests, variations on preparation and activation cycles and more sleepless nights, the company finally achieved success using the lower phosphorus chemistry. Not only was this a huge victory for the customer, but this incident provided insight into previous situations where the electroless nickel deposit had failed.
Several “small” changes were made to the existing process cycle to produce success. Now the focus was placed on the substrate, cleaning and activation portion of the SCRAP process. The existing process was mentioned previously, and it is noted that many shops do not have the luxury of this cycle, which is sad because it represents a good electroless nickel starting cycle.
Essentially, five changes were made to the existing process sequence:
Because these high-carbon parts were not oily, having been precleaned at the machine shop grinder, they only exhibited the effects of the heat scale tarnish. As a result, an alkaline descaler chemistry in the primary soak tank and the anodic electrocleaner was employed. Two different types of descaler chemistries were used in the soak and electroclean stations. In the soak clean, an alkaline permanganate descaler was installed. In the electrocleaner, a high-alkaline potassium-hydroxide-based electrocleaner was used. Adding a rinse between these stages was necessary to remove any of the permanganate. Still, it is good practice to install a rinse between any type of soak clean and electroclean to improve the efficiency of the cleaning process while increasing the life of the electrocleaner.
Additionally, adding an ammonium hydroxide predip to the process ensured that the electroless nickel deposit initiation took place quickly. This predip also assisted with the removal of any surface films that might have been present. After all, ammonium-based window cleaners work well to remove surface film residues, and why should this case be any different? Post-baking the parts improved the adhesion to the substrate. Additionally, prebaking the parts for a stress “relaxation” was essential to promoting the best adhesion. The modified process sequence is as follows.
Electroless nickel plate
This explanation can get very complex and will almost surely spark disagreement among metallurgy experts. Disagreement is all right since the end results were acceptable after making the necessary changes to the pretreatment process.
It has been noted that as deposited electroless nickel coatings are not in their equilibrium state2. A simple way to view the problem is explained in the following interactions. These interactions can help to explain some of the relationships that are seen with preparation practices. As one decreases the phosphorus content from 10 to 6% w/w, the “lack of deposit ductility” increases. The ductility improves as phosphorus content decreases further from 5 to 1% w/w. Also, “lack of ductility” can be related to the asplated deposit’s microhardness, which also increases with decreasing phosphorus levels. Another way to view this is that the elongation of the as-plated 5-6% phosphorus deposits tends to be less than the elongation for higher phosphorus deposits. There is also a relationship to the deposit microhardness, which is influenced by the crystallinity or lack of crystallinity of these deposits.
There are many papers that discuss in detail these relationships and their interactions2, 3, 4. Through many xray diffraction studies, these papers show various phases do exist with electroless nickel, and that the properties of these phases are related to the phosphorus content. It has also been noted that when electroless nickel coatings are completely in their beta or gamma state, they are in their most homogeneous and relaxed state5. Internal stresses occurring between the substrate and nickel phosphorus deposits also impact the ultimate adhesion. It is not important to discuss the details of these interactions here, but simply understand there are many other relationships that ultimately impact success or failure.
Given all these interactions and variables, especially when plating over heat-treated, high-strength or high-carbon substrates where scale is evident, complete removal of the heat scale is necessary to allow complete atomic bonding of the nickel deposit to the substrate. The atomic bonding becomes more crucial as the deposit’s microhardness values decrease and as the percent elongation decreases with lower phosphorus levels. Where complete removal of scale is not achieved, poor adhesion can result from these differences and their interactions with respect to elongation, deposit microhardness and the internal stresses exerted from both the nickel phosphorus deposit and the substrate.
In this case, the customer had not been successful removing all of the heat scale. Therefore, the high-phosphorus deposit was more forgiving due to its structure, internal stress, elongation and microhardness interactions with the existing preparation line. Switching the deposit to the lower phosphorus chemistry changed the deposit structure, internal stress, elongation and microhardness so that the proper atomic bonding would no longer exist over the remaining heat scale. The choice was simple, find a new preparation cycle or switch back to the high-phosphorus deposit.
Now the entire electroless nickel plating system required a modification to rebalance so a new equilibrium with the preparation cycle could be found, especially if the lower phosphorus system was to be used. This explanation represents an oversimplification of all the interactions, but one cannot argue against the successful results obtained by employing the strategies of SCRAP in this situation. The customer also benefited from the increased plating speed, faster throughput and reduced costs.
A process cycle for a new part or substrate can be fun and interesting to develop since one must balance producing a robust, practical cycle that is easy to use with an economical cycle. It often can be the thrill of victory and the agony of defeat. Although the individual chemistries within any cycle used are based upon science, there is still a lot of experimentation and adaptation involved in the cycle development process. Some have said this is a fancy way of pointing out that trial and error still exists in cycle development. This is not a point for disagreement, since existing cycles designed for specific substrates require some trial and error based upon science to see what will perform the best.
A customer had been approached by a manufacturer of neodymium substrates to plate electroless nickel. These types of substrates have traveled around our industry for the last 10 years with some success and much failure. What makes these substrates unusual is that they contain primarily neodymium but many other rare earth metals as well. They are difficult to process because they are pressed into unusual shapes, and, because of being pressed, they can have a lot of porosity. Additionally, the other contributions from the rare earth elements can vary both in type and orientation within the alloy. This causes difficulty with the process cycle due to unusual amounts of smut formation during certain process steps. So how is it best to design a cycle for these materials given the restrictions?
The process development cycle includes several steps:
In this case, the plated parts were required to pass a 168 hr salt spray test and a high-temperature, 95% relative humidity environment test.
To assist with these determinations, some tools were required. One tool that worked well was an instrument that used photo electron emission (PEE). This can be used to troubleshoot cleaning or rinsing problems. Based on optically stimulated electron emission (OSEE), this instrument is beneficial in the development of specific cleaning schemes for a particular part or substrate8. The instrument functions by illuminating the surface with UV light, which interacts with the surface to emit electrons. These electrons are collected and converted into a voltage then displayed as a value. A clean surface, as recognized by the monitor, emits the maximum amount of electrons while a contaminated surface attenuates the electron flow from the surface and results in a lower value. Basically, the clean surface produces a high reading, and a soiled, smutted or contaminated surface produces a low reading. The values are used successfully to determine relative differences in the surface quality.
In this case, it was a good assumption that highly alkaline cleaners would attack the substrate and result in the production of smut. For this reason, the initial benchmarking process determined that lower alkalinity cleaners provided the best option for cleaning this substrate. It was determined that those used for aluminum processing worked best to clean the surface while providing a free rinsing surface based upon OSEE evaluation readings.
The first step was to determine what types of acid or alkaline activators would either attack the substrate or cause excessive smut formation. The approach here was to qualify the best options. In addition to using the OSEE process to see how the surface might form smut, an additional surface profilometer was used to measure the surface roughness created by the different activation processes. This is an effective way to determine substrate attack and helps to quantify how much occurs. A test matrix was set up, and the relative surface finishes were examined after exposure to various activation schemes. The results of the testing showed that chloride and fluoride attacked grain boundaries of the substrate. Because of this attack, there was some smut formation on the surface, but a mild anodic electroclean activation removed the smut. Additionally, the attack of the grain boundaries created a weakness within the alloy that contributed to salt spray/humidity test failures within 48 hr, even at nickel deposit thicknesses greater than 0.0005 inch. An alternative approach was evaluated that did not produce any smut on the surface. Without smut, the reason for using an electrocleaner was dismissed because OSEE readings did not indicate that an electroclean produced any advantage in cleanliness or contribute to the success of the process cycle.
Due to the excessive porosity of the alloy, various electrolytic strikes were used in an attempt to encapsulate the substrate. The use of the strikes also helped produce a duplex nickel layer barrier effect. It was shown that a modified sulfamate nickel chemistry produced the desired result without detrimental effects on the alloy. The Watts, Woods and modified Woods strikes all contained high levels of chloride that produced poor results with regard to corrosion performance, even though they produced good adhesion.
What were the conclusions of all the testing? The resulting process cycle was shown to produce all the necessary requirements. Adhesion, appearance and environmental exposure were all acceptable.
Post treatment and dry
This same cycle was attempted on another neodymium substrate at a later date and would not produce the same results. In that case, the substrate had been impregnated to seal the porosity and those residues were present on the surface. As a result, an alkaline descaler chemistry based on potassium hydroxide was used as the soak clean to successfully process the parts. That was the only chemistry that would adequately remove the residue. OSEE testing again confirmed this was the best route to take in that situation.
Through these case studies, it was shown how electroless nickel plating problems can be traced back to a relationship between the substrate condition and how well it reacts or acts in the preparation cycle. Paying attention to the concepts of SCRAP will provide good strategies for producing quality electroless nickel deposits. The importance of preplating operations cannot be overemphasized when attempting to produce quality electroless nickel deposits. Having the best electroless nickel solution or system will not change the fact that a poor surface preparation will cost the finisher more time, more money and more headaches in the pursuit of quality. It is hoped that this information when reviewed with other papers on the subject will provide guidance and insight to producing acceptable deposits and in solving problems on the electroless nickel plating line.