Cleaning for Plating Success

Many methods can be used to enhance the surfaces of materials with metal finishes. Given the versatility of electroplating, a wide variety of properties can be created by adding a layer of metal or two: appearance, corrosion protection, electrical properties and wear resistance, to name only a few. But all of these examples have one thing in common: If the components are to maintain the their functionality over an extended lifetime, the adhesion between the substrate and the metal coating has to be strong. So good surface preparation may be invisible, but it is the key to electroplating success.

But there are a lot of substrates, and a lot of potential contaminants on parts to be plated. For machined parts, today’s manufacturing practices may use specialized lubricants to enable precision tolerances. Cast iron or zinc may have to be polished before plating, which can leave buffing compound residue on the part. Casting or injection of aluminum, magnesium or various engineering plastics can leave mold release agents on the part surface. These challenges are what makes selection of suitable pretreatment chemistry and process cycles essential steps for the successful function of any manufactured component.

Concepts for the cleaning of parts

The cleaning of parts in the plating process can typically be divided into three basic steps.

1. Immersion Soak
2. Electrolytic Cleaning
3. Acid or Alkaline Activation

Immersion soak cleaning  

First, all organic and inorganic surface contaminants must be removed. Generally, these contaminants are dissolved and removed in chemical cleaning solutions. Owing to the large number of contaminants, there are various systems available. These include both highly emulsifying hot degreasers and weakly emulsifying (de-emulsifying) or paste-dissolving systems. These aqueous degreasers can remove many oils and soils.

In days gone by, cleaning agents such as Nonylphenol Ethoxylates (NPE/APEs) might have played a role in these cleaning steps, but today they are prohibited in many regions and their use is far less common. This has challenged formulators to find new wetting agents and surfactants for these cleaning systems. And because a production is only making money when it’s running, they must deliver effective cleaning on the various surfaces and a long solution life at the same time.

Before the right choice can be made, the essential properties of both the part and the contaminants must be known.

• Nature and composition of the substrate (alloy identification)
• Level and type of contamination present on the surface (soils, oils, etc.).  There may also be alloy constituents that must be removed during the cleaning process
• Parts with complex geometry, especially those with deep recesses or blind holes, may require special handling for rinsing, racking and draining
• Applicable specifications and OEM requirements that the finished part must meet
• Available equipment and process performance requirements
• Waste treatment and environmental compliance

These details are sometimes unknown to the plater; he or she just receives a part and knows little about its preparation. In these cases, the right cleaning system has to be determined in a series of trials for the substrate. It is necessary that the type of soil be characterized prior to commencing the pretreatment cycle.

Surface contamination can be divided into three general categories:

1. Those due to the metallurgy of the base metal and any nonmetallic inclusions in the base metal that might be present
2. Organic soils — mineral oils, animal fats and vegetable oils used as lubricants in the manufacturing process
3. Inorganic films — scale from a heat treatment operation, rust or tarnish
    

Electrolytic cleaning

The second step involves electrolytically assisted fine cleaning to produce a clean, active surface. Electrolytic degreasing can be:

• Cathodic — soils, smuts and metallic ions are attracted to the substrate
• Anodic — positively charged ions and smuts are pushed away from the substrate
• Periodic Reverse — a cyclic reversal of electrical polarity on the substrate to push and pull contaminants off the surface

Depending on its composition and application, the degreasing agent can also remove thin oxide layers and coatings from surface finishing processes. An electrocleaner creates its own agitation with gases derived from the electrolytic decomposition of water. The electrocleaning mechanism uses the features listed for soak cleaners and adds the following:

• Mechanical Scrubbing — supplied by gas evolution at the metal surfaces
• Mixing the Surface — the turbulence at the metal surface during mass transport
• Water Softening — preventing formulation of insoluble calcium and magnesium salts
• Metal Chelation — the solubilizing of metallic ions such as copper and iron into the cleaner and preventing redeposition onto the part

Each treatment step is followed by a multistage, cascading rinse process whose effects are enhanced by sprays or agitation to ensure complete rinsing.
 

Acid or alkaline activation

The last step before plating uses acid or alkaline solutions to activate the substrate and remove any rust or mill scale that was not already removed from the surface. These solutions can also neutralize the surface pH of the part from any effects of the previous steps. The activation step is the most alloy-specific step in the process as certain alloys respond to acids and mixtures of acids in different ways, and the right balance must be found. The goal is to clean the surface, not change it.

Hydrochloric or sulfuric acid is most often used for acid cleaning of steel. These acid solutions are effective in removing rust and scale (but not soil) from the metal surface. However, there must be some care, as indiscriminate use can increase the potential for hydrogen embrittlement as well as base metal attack. 

Due to the variety of alloys available, the cleaning of aluminum parts is significantly more complex. Selection of an aluminum cleaning and activation process must start with identification of the alloy, then consider the subsequent processing needs to decide how to properly prepare the surface.

 

Facility cleaning lines

Modern plating installations offer at least two different hot immersion degreasers. Many also include a cathodic degreaser that can be added to the process when necessary. They can also involve several pickling solutions. For maximum versatility and quality, plans for a new plating installation should consider the space requirements and investment costs needed to cover the range of substrates and contamination types your customers may bring you.

Another useful approach can be the use of a product like MaxPro NNP.  This Near Neutral Pickle can handle many of the cleaning chores that required a strong acid pickle, but with far gentler conditions. This means the range of substrates it can treat without damage is much wider, and this may save you a tank or two (not to mention processing time).

Process control

Most cleaners will do a pretty good job when new. But industrial production means they need to work under continuous use, and this means that process control can’t be an afterthought. The classic acid-alkali titration of cleaners can be supplemented by other quality assurance methods such as test inks for measuring the surface tension of cleaned components. This will let you know if your parts are clean. And directly measuring the surface tension of cleaner solutions enables you to check the active wetting agent concentration in a cleaner quickly and with relatively little effort. This lets you respond faster to changes in the process and avoid rework or scrap.

As mentioned above, the choice of cleaners is often a matter of experience and may require selection tests in a plating installation. If the contaminants or the lubricant used in processing the metal is known, a preliminary selection of the cleaning system can be made under laboratory conditions. This involves applying the lubricant to a clean test piece, testing various systems and comparing the results. Often, the returned laboratory findings can simply be transferred to a plating installation. In addition, these tests are a highly convenient method for showing that different cleaning technologies remove contaminants in different ways. This makes clear whether it is necessary to operate two separate hot degreasers with differing systems. Figure 2 is a good example. It depicts the cleaning effects of different systems on a test oil. Dark discolorations after rinsing and drying are clear indications of residual soiling (incomplete cleaning).

New requirements for stripping racks

In the wake of growing requirements (such as corrosion resistance) for plating systems, recent years have seen a rising trend in the use of intermetallic layers. Of special mention here are the alkaline and acid zinc-nickel methods. Removing these layers from rack hooks or reworked parts may leave a dark residue on the surface. This residue contains nickel of low solubility that cannot be removed with conventional inhibited pickling. If the residue is not removed, future parts may encounter problems specifically at the rack hook.  Alternatively, it may be necessary to take extreme measures to clean the racks, such as ultrasonic assisted rinsing or cleaning.

A high level of productivity and quality can be maintained with the use of new high-performance inhibitors that have been created specifically for this purpose. The preferred method for stripping rack hooks or reworked parts is to introduce them to a first pickling solution that contains these high-performance inhibitors to prevent a buildup of metals in the main pickling solution.

Process technology can extend cleaner lifetime and effectiveness

The biggest challenge to consistent cleaning is often the accumulation of contaminants in the cleaner tank itself. Physical separation of contaminants can significantly improve the lifetime and stability of a cleaning process. Process aids like gravity oil-water separators or oil skimmers are well-established options to consider. They are simple solutions to prolong solution life of a hot degreaser and to minimize the waste treatment costs.  

Some membrane filtration methods can offer even higher removal rates of contaminants, although higher removal rates must be balanced against the functional lifetime of the filters and the accidental removal of cleaner chemistry itself. But when the largest cost is the need for frequent new makeups of the hot degreaser, any measure that extends the solution life will keep cleaning performance consistent and save chemistry costs.

Another process factor that can enhance the value and consistency of your cleaning operation is good solution movement in the cleaning solution. Modern long-life lubricants are often highly viscous and can be effectively removed only with additional mechanical movements and/or bath agitation. Eductor/Venturi systems for solution movement and 2D or 3D cathode movements can be installed to maximize efficiency of the cleaner.  

And, finally, the most important parameter in a hot degreaser is the operating temperature. In most cases, good cleaning results are hard to get in an unheated solution, particularly for viscous contaminants. For traditional cleaners and tough cleaning challenges, the tank heater must be designed to reach and hold temperatures higher than 70°C. However, new low-temperature cleaners are becoming available that will enable operation at temperatures as low as 50°C with equivalent performance, and these offer big savings in energy and safety.

Summary

A clean surface is the foundation for high-quality electroplating and durable surface finishes on high precision parts. Given the wide range of contaminants found on incoming parts and the wide variety of substrates used for their construction, it is important to have a partner who understands how to optimize cleaning and move smoothly into electroplating steps. This is why we think surface preparation is every bit as important as plating chemistry. And with the increasing cost of waste treatment and energy required to run a cleaning process, there is no room for any waste of time, material, space or heat in your shop. This is why we offer a wide range of surface cleaning options, along with the expertise required to help you design the right process for your business. But it doesn’t stop when the cleaner tank is filled. With automated replenishment, wastewater treatment options, analytical methods and process control strategies, we also help you achieve stable and dependable results for the long term. That clean surface gets covered up by a valuable electroplated layer, but that invisible foundation is key to making the whole component strong, beautiful and durable.