Question: For years, we have been receiving mechanically polished non-ferrous metals that we nickel and chrome plate in a rack system. The high-gloss nickel layer is around 20 µm, while the chrome layer is of a thickness that is usual for this purpose. The process has been perfected over the years. Apart from the fact that we switched to a chromium(III) electrolyte around two years ago, there have been no significant changes.
About a year ago, a series of complaints began that we are still dealing with today. These are massive adhesion problems. The nickel coating is peeling off over large areas, particularly in the highly visible areas of the parts. Both our customer and our chemical supplier claim that they have not changed anything, and we cannot detect any changes in our parameters either. The pre-treatment was completely renewed several times, the nickel electrolyte was analyzed, cleaned and even reapplied in the meantime. However, there was no improvement. What options do you see for finding and eliminating the cause?
Answer: It is well known that most problems, especially with regard to adhesion, come from pre-treatment. In electroplating, this is said to be between 60 and 80 %. However, the question always arises as to what extent a classic pre-treatment can or must be able to eliminate problems with the material and surface condition. This mainly concerns challenges that electroplating is unaware of because they are not visible on the surface. In addition, you have certainly perfected your process to a certain initial state. The use of chemicals, times and temperatures has been optimized over the years, which has a positive effect on throughput and costs. The problem with such optimizations, however, is that they no longer deliver sufficient results across the board in relation to various surface conditions. The greater the fluctuations in the initial state, the more frequently complaints occur. Put simply, in this case a process has been over-optimized and is no longer able to sufficiently electroplate a normal range of surfaces.
Figuratively, you can think of it like a set of scales where the individual parameters, depending on their weighting, cause the scales to tip in the direction of "OK" or "not OK". O.". In the worst case, you have a physical pendulum which, when it is up, you can no longer tell which side it will fall to as soon as you let it go. It is in a state of maximum sensitivity and every little thing can decide whether the result will be good or bad.
Since you are talking about non-ferrous metals (Fig. 1), we have to assume that you are talking about different copper alloys and not a specific alloy that causes problems. Furthermore, you do not indicate whether one of these alloys is particularly conspicuous. Overall, it is in the nature of things that we can only provide very general information. You will therefore have to carry out some investigations yourself to find the causes. We will try to evaluate the individual parameters roughly according to probability so that you can prioritize further investigations.
Alloys
It is not very likely - but also not completely impossible - that something has fundamentally changed in all alloys, especially if it is possibly predominantly a certain alloy that is causing the problem but has never been looked at more closely.
You will certainly receive a large number of different items from the customer, but they are mainly made of the same material. Our experience shows that this may not be correctly stated by the customer to the electroplating company. Therefore, you cannot know for sure whether they are different alloys or not. The drawings, if any, or the order papers may simply state "brass" or "bronze", but this is an inadequate description.
It is important to clarify this. In addition, your customer should provide information on whether the supplier has been changed in the meantime. The entire process chain from the foundry onwards should be considered. You may be able to obtain information from the foundry about the relevant composition.
Even if you receive the information, you should carry out a cross-check on receipt of the goods, preferably using the X-ray fluorescence method. In this way, you can quickly obtain information as to whether you are actually receiving what the customer specifies.
If you have sufficient data, you can compare it with the "n. i. O.' parts and check whether the problems are limited to a certain type of alloy or a specific alloy. If this is the case, further investigation into the chemical composition and physical properties should be carried out to determine if there are any unusual properties. It is also possible that the alloy and properties are within the normal range, but the "optimizations" of your process do not provide sufficient process reliability for this material.
It may be helpful to reconsider and adjust the changes in the specific process parameters to ensure that they are also suitable for the particular requirements of this alloy. Additional steps in the pre-treatment or changes in the coating parameters may be required to address the adhesion issues.
This requires time, effort and ultimately communication. Experience has shown that this has already solved some problems "by itself", as all parties involved have been sensitized prior to electroplating.
Influences on the surface condition
All process steps have an impact on the condition of the surface. This applies to machining operations such as turning, milling, grinding, scraping, brushing, etc., heat treatment, forming and, of course, polishing (Fig. 2), which appears to be the last step before electroplating. The interaction with a modified alloy should not be ignored. Higher lead contents, for example, can lead to massive smearing, which makes electroplating more difficult or - at least in processes that are not designed for it - impossible.
Fig. 2: Metal polishing robot in an Italian company (Photo: stock.adobe.com/Giovanni Burlini)
Our experience shows that mechanical companies are concerned with adhering to the specifications on the drawing. No attention is paid to galvanizability. This means that parameters can change massively here without the effects on a possible coating being considered, as the result meets the requirements of the drawing. In addition to the dimensions, this also includes roughness and gloss level.
Depending on the process, the changed parameters can include feed rate, pressure, times, temperatures, lubricants, speeds, etc. For the mechanical processor, the aim is to increase efficiency. They are often not even aware that this also changes the condition of the surface. This can have very different effects on the surface. A thicker Beilby layer [1], a much thicker oxide layer in general, denser surfaces, overlaps, cracks, pores, embedded dirt and much more. Unfortunately, these possible causes are difficult to recognize even on the micrograph - sometimes not even detectable. This is because the cause of adhesion problems has often been removed or at least concealed during pre-treatment. In addition, it is difficult to pinpoint the exact point at which the cause of the problem becomes visible when examining materials.
A sensible but time-consuming approach is to take sufficient retention samples so that a direct comparison can be made later. Depending on the complaint rate, however, this is often not possible with just a few parts. Added to this are the high costs for microsections and SEM, which is why many electroplating companies are reluctant to carry out such tests. As a rule, however, such costs are much lower than further complaints.
Polishing
As this is the last mechanical step before your electroplating and experience has shown that some errors can occur here, we would like to look at polishing separately.
Like the abrasives, the polishing powders can also be mixed with greases and waxes etc. and processed into molded bodies, which are then used as polishing pastes. Basically, grinding and polishing pastes, i.e. solidly bonded moldings, are used exclusively for hand grinding or polishing. In some cases, solid pastes are also successfully used on automatic machines with the aid of paste feeders. Polishing emulsions, on the other hand, are used exclusively on automatic machines (Fig. 2).
In some cases, solid pastes are also used successfully on automatic machines with the aid of paste feeders
Polishing emulsions are more or less viscous liquids consisting mainly of oils, greases, polishing powders and water. This mixture is prepared into the actual emulsions with the help of emulsifiers. These emulsions are applied to the wheel or metal surface at intervals using spray guns with electro-pneumatic control. Dosing can be more precise than with manual application or with feeding devices for solid pastes.
In addition to conventional grinding and polishing pastes, so-called water-soluble pastes are also available on the market. Strictly speaking, the term "water-soluble" is not entirely correct, as all grinding and polishing powders are neither soluble in organic solvents nor in inorganic cleaners. The term "water-soluble" merely means that the polishing dirt - comparable to soap - can be emulsified and removed in water, preferably warm water [2][3].
Temperatures of up to 1000 °C can be reached during grinding, brushing and polishing
While the inorganic part - the grinding and polishing powders - is absolutely resistant to the temperatures that occur during grinding and polishing and does not change, the organic part, i.e. the binders consisting of fats and waxes, is sensitive to excessive heating. These organic substances begin to decompose or burn at certain temperatures. This usually results in the formation of carbon-rich compounds that are no longer soluble, or at least partially insoluble, in organic solvents, much less in water or inorganic or alkaline cleaners, and sometimes even in acids. If the grinding or polishing process is too sharp or too hot, the organic part of both the grease-containing and the water-soluble pastes may react and burn during the grinding or polishing process. As the metal surfaces become softer at the relatively high working temperatures, the burnt organic part can roll into the surface [4].
It should be clear to any expert that such a surface can no longer be cleaned properly, as the dirt is "ironed" into the surface. Such surface defects should be clearly visible under a microscope with sufficient magnification.
Corrosion protection and storage
Problems often occur due to corrosion protection agents and storage or storage times. Benzotriazole (BTA) is often used as a temporary corrosion protection agent for non-ferrous metals and silver. This rarely causes problems in electroplating shops, as BTA can be easily removed by acids and anodic degreasing in particular.
Changing the corrosion protection agent, on the other hand, can lead to massive problems, as new developments are often only tested for the longevity and durability of such molecules, but not whether the surfaces can subsequently be electroplated.
Of course, the reverse can also occur. Inadequate corrosion protection, poor storage conditions and long storage times can lead to corrosion, even if this is not always visible. One possible consequence would be that your "optimized" process is no longer able to remove the somewhat thinner oxide layers, which can lead to the aforementioned error pattern. It is conceivable that pickling processes were not used in the past because the surface condition was very good, possibly also because the goods were not stored at all and were sent directly to the electroplating shop. If the customer did not use an anti-corrosion agent during this time for precisely these reasons, but the goods are now being stored without appropriate protection, this may well be the cause of your problems.
Pre-treatment
As you have already noted, numerous pre-treatment measures were taken without solving the problem. This could indicate that the cause is not the pre-treatment itself, but that it may no longer provide a process-safe coating for the material supplied.
Nowadays, optimizations focus primarily on costs and less on process reliability. In particular, the parameters that destabilize the process are "optimized". The temperature is lowered, the concentration reduced, components are replaced or even omitted altogether. Unfortunately, such measures often result in process capability being certified after a short period of time without considering the long-term effects. This becomes particularly problematic if the initial state of the material is in a "perfect" condition at the time of the changes. If the pendulum swings in the other direction, the measures taken can no longer compensate for such fluctuations.
In the hope that all process changes have been well documented, we recommend critically reviewing the list of optimizations made in recent years. A radical step could be to reverse all changes and check whether the process can be stabilized as a result.
Of course, the reverse is also possible: substances are added - sometimes even without corresponding documentation. A concrete example would be sodium silicate, which is added to degreasing solutions because it has a high dirt-carrying capacity and at the same time protects non-ferrous metals from attack by hydroxides. It is quite common for this agent to be shoveled into the degreasing system according to the "a lot helps a lot" principle. Despite all the advantages, this has the disadvantage that silicates are deposited on the surface and are no longer sufficiently removed. This may require a fluoride-containing pickling or decapping agent, which in the past may have been replaced by a more cost-effective hydrochloric or sulphuric acid decapping agent.
A problem in connection with modified polishing agents or modified parameters during polishing can arise during degreasing. Temperatures of up to 1000 °C can be reached during grinding, brushing and polishing. The resulting heat is dissipated by greasy or synthetic substances. In the past, grinding and polishing pastes usually contained saponifiable animal fats, such as beef tallow. Today, for cost reasons, they are often diluted with unsaponifiable, only emulsifiable paraffin-based hydrocarbons. However, a saving in the purchase of pastes often has to be paid for with an increase in pre-treatment costs. At higher processing temperatures or longer storage times of the workpieces, paste residues can harden due to oxidation or polymerization and make it more difficult to remove them.
Liquid or paste-like grinding or polishing emulsions often contain oil-soluble emulsifiers or dispersants, which can impair the effectiveness of some surface-active substances contained in degreasing solutions. In order to prevent the separation of liquid grinding and polishing emulsions during longer storage times, high-molecular compounds are often added to increase viscosity. These can impair water rinsability and cause haze or clouds during subsequent electroplating. In the worst case, they can even lead to adhesion problems.
Rinsing
Another common cause, possibly even interacting with previously mentioned potential triggers such as sodium silicate, is rinsing. Problems of this kind occur, among other things, when rinsing steps are "optimized away", the rinsing criterion is generally not achieved or circulation water no longer has the required quality. Although the latter is monitored via the conductivity, there are impurities that do not affect the conductivity. One example of this is numerous organic compounds that temporarily accumulate in ion exchangers and are then released into the rinsing process.
Aging systems suffer from bacterial infestation, especially in the sink area. Depending on the type of bacteria and their concentration, they can cause massive problems with the coating. A preferred place for bacteria, for example, is the cross-transfer unit, which is rarely cleaned and completely replaced due to its volume. Some bacteria are even able to oxidize copper, which in this case could lead to them introducing a passive surface into the nickel electrolyte [5]. Depending on the system and control, the effect is intensified by the fact that some product carriers have a longer dwell time in the sink before nickel plating. In borderline cases, even activation directly before the nickel electrolyte may no longer be sufficient.
Nickel electrolyte
As you have stated that the nickel electrolyte has been newly prepared, it could be assumed that this is not the source of the problem. However, attention should not only be paid to the chemical composition and purity.
The cleaning of the container, the anode rods and the anodes themselves also play an important role in new preparations. The same applies to filters and possibly other pumps, air injection and selective cleaning, provided this is connected directly to the electrolyte. Anything that has to do with power supply can cause problems that can lead to a passive surface and thus to adhesion problems, for example. This applies not only to the anodes in question, but also to the cathode rods, power supply, rectifier and, of course, the racks.
By labeling the racks and product carriers, you can quickly find out whether they are more likely to cause the problems mentioned. An indication of problems with the anodes could be the distribution of the "n. i. O." parts on the product carrier. In addition, if there are several stations, attention should be paid to whether the problematic parts always come from one or more specific stations.
Even if this is not the case, you should still have all rectifiers checked by the manufacturer, as well as the system control. Always question whether the values displayed are actually correct, even if these values correspond to expectations.
With regard to the chemical composition, we are aware of cases in which the chemistry itself has not been changed, but its concentration has. It may well happen that you are now supplied with a more concentrated version of the gloss additive, but this has not been communicated. Massive overdosing then also leads to the aforementioned error pattern, and the problems may also be related to the dosing intervals and the type of dosing. This can happen in particular if new and therefore inexperienced employees dose manually at the wrong point or even in the wrong concentrations. You should also take this opportunity to check whether new employees have been working on the system since the problems started. The human factor should never be ruled out when troubleshooting.
Further considerations
Similar to the nickel electrolyte mentioned above, passivation can also occur in an electrolytic degreasing process. This is rather rare, but should not be ignored.
Errors can also occur if the racks are loaded without gloves. In certain cases, hand perspiration can be the source of the problems mentioned.
A common problem is multiple causes that can lead to the same undesirable result. For example, a combination of overdosing and poor contact or a critical material and bacteria. When investigating causes, one tends to take a step-by-step approach to isolate and fix the problem. However, if several causes accumulate and time elapses between measures, it becomes almost impossible to solve the problem. In such a case, you have no choice but to carry out a complete restart. This means that the system is completely drained, cleaned and serviced before it is restarted to ensure that all the possible disruptive factors mentioned above have been eliminated. This, of course, requires the reversal of all optimizations that could have an impact on quality.
Literature
[1] The mystery of the Beilby layer; Galvanotechnik (2023), 7
[2] Practical electroplating technology; ISBN 978-3-87480-277-2; Eugen G. Leuze Verlag
[3] The testing of polishing, lapping and grinding media; ISBN 3-87480-074-1; Eugen G. Leuze Verlag
[4] Handbook for grinding and polishing; ISBN 3-87480-021-0; Eugen G. Leuze Verlag
[5] Microbiological treatment of waste water from the metalworking industry and recovery of heavy metals; Galvanotechnik (2000), 10
