Surface Preparation

Surface Preparation Services

Poor results of an electroplating or electroless plating procedure can often be traced to one factor — the failure to properly prepare the substrate surface. Unfortunately, too many metal finishing operations overlook the importance of the preparation/pretreatment step.

The presence of dirt, grease, oil, rust, chemicals and other foreign materials can negatively impact coating adhesion, durability and continuity, and must be removed prior to the application of the metal coating. The best metal finishing companies understand the quality of an electrodeposit is only as good as the condition of the underlying surface.

Key Surface Preparation Factors to Consider

There are several factors to consider when implementing surface preparation for plating. First, it is necessary to properly identify the composition of the base material. Each metal (or other material type such as plastic or ceramics) will react differently to the various preparation methods; it is important to ensure the chosen pretreatment procedure will effectively prepare the surface without damaging it.

The second factor is the correct identification of any foreign materials that may be present on the substrate surface. This will help to determine the most effective and safest method for removing the debris. The final factor is the evaluation of the type of metal that will be used to plate the substrate and ensuring it is compatible with the types of surface preparation methods selected.

Overview of the Surface Preparation Process

While there are many variations based on the existence of the factors listed above, metal preparation for plating is essentially a three-step process consisting of pre-cleaning, secondary cleaning and surface activation. We’ll now take a closer look at each process phase.

Pre-Cleaning

Surface preparation for electroplating typically begins with pre-cleaning, which usually entails soaking the substrate in some type of cleaning agent. The soaking loosens and removes the bulk of the foreign material from the surface. The metal part or component is submerged in a tank filled with a hot alkaline cleaning solution, the strength of which is determined by the composition of the materials that must be removed. These cleaners are generally categorized as:

• Light duty: Light duty soak cleaners include materials such as wetting agents, buffering salts, dispersants and inhibitors. Light-duty cleaners normally work well for removing lighter contaminants and soils from steel, aluminum copper and zinc base die castings, without causing etching or tarnishing.
• Heavy duty: Heavy-duty soak cleaners contain a balanced blend of highly alkaline builders, silicates and sequestering agents. They are used in a wide range of general-purpose cleaning applications involving the removal of contaminants such as fatty oils, grease, drawing compounds and corrosion preventatives.

Secondary Cleaning

Secondary surface cleaning, also referred to as polishing, removes any remaining microscopic soil/contaminant traces that were not eliminated during the pre-cleaning phase. The various secondary cleaning processes include:

Abrasive Blasting

Abrasive blasting involves the propelling of a pressurized stream of abrasive media onto the surface of the substrate. When executed properly, the abrasive blasting process can forcibly and effectively remove various types of contaminants, but without damaging the substrate surface. The wide range of available blasting media allows for the tailoring of the process to the application. For example, steel shot media works best for polishing and peening tasks, while steel grit is the better option for stripping off contaminants.

Ultrasonic Washing

Ultrasonic washing is a preferred option for high-precision metal parts cleaning applications. The parts are immersed in a specially designed tank containing an alkaline, acidic, solvent or detergent-based cleaning solution. The next step is the introduction of high-frequency soundwaves to form cavitation bubbles that scrub off service contaminants.

The cavitation bubbles can drive packed dirt and contaminants from deep or blind holes in the surface. To be effective, ultrasonic washing requires the appropriate selection of cleaning agents, maintaining the proper solution temperature and achieving the right balance between the ultrasonic power level and liquid solution volume in the tank.

Spray Cleaning

Spraying is a simple yet powerful cleaning method for virtually any type of large metal surface. Spray cleaning entails the application of heating chemicals via spray nozzle. The combination of thermal, mechanical and chemical energy provides the strong cleaning action. It is also possible to alter the effect by varying the nozzle pressure and spray pattern, as well as the temperature and volume of the chemical cleaning solution. The typical cleaner formulation used in spray cleaning is a mildly alkaline, light-duty solution combined with low-foam wetting agents.

Pickling

Picking is used to remove surface impurities such as scale, rust and inorganic contaminants from ferrous, or iron-containing, and precious — gold, silver, platinum, etc. — metals, as well as aluminum alloys. The pickling process involves immersing the metal part into a solution known as pickle liquor, which primarily consists of hydrochloric acid.

Pickling offers an effective method for remove an oxide layer or scale from a metal surface. Acid cleaning methods such as pickling must be used with extreme caution due to the corrosive nature of the acid. In certain types of metals and high-carbon steels, the hydrogen from the acid can react with the metal surface, resulting in hydrogen embrittlement.

Electrolytic Cleaning

Electrolytic cleaning is a popular choice for applications where surface cleaning and activation are desired. Commonly referred to as electrocleaning, this process involves the introduction of a controlled DC electric current into an alkaline cleaning solution containing the immersed part.

The specific alkaline formulations used in electrocleaning include:

• Standard alkaline cleaners, which work best on steels, copper, nickel and nickel alloys
• Inhibited alkaline cleaners, which are most effective on brass, bronze, zinc and zinc alloys, tin, and aluminum and its alloys
• Acid-based cleaners, which are the preferred choice on magnesium

Surface Activation

In some cases, it may be necessary to “activate” the surface after cleaning. Surface activation is especially important when plating metals onto non-metallic “exotic” materials such as plastic, glass or ceramics. The three primary surface activation methods are plasma treatment, chemical activation and ultraviolet radiation. Specific purposes for surface activation include:

• Electrical conductivity: Certain types of metals and most exotic materials are not good conductors of electricity. The implementation of plasma treatment or UV radiation can increase the volume of loosely bonded or free electrons on the surface, which enhances conductivity.
• Adhesion: Surface activation can prevent materials such as dust or carbons from adhering to the surface of the substrate, which provides a stronger coating and a more durable finished product.
• Metallization: The metallization of plastics and polymers allows them to serve as an effective replacement for copper, aluminum and other metals in electrical appliances. This can extend the product’s lifespan by reducing the likelihood of rusting.
• Creating a diffusion barrier: Exotic materials are susceptible to damage from atmospheric gases such as methane and nitrogen that can penetrate the surface. Activation will alter the atomic structure of the substrate surface, creating a defensive barrier that will protect against diffusion.
• Fogging reduction: Polymers such as nylon and polypropylene can develop a “foggy” appearance due to the trapping of water by hydrocarbons on the surface. Chemical activation can create an assortment of functional groups on the surface that will reduce or even prevent fogging.

The Goals of Surface Activation

Choosing the appropriate surface activation method depends on several factors. The composition of the material is a crucial consideration. Each type of material has its own unique set of properties that affect its reaction to a specific activation process. For example, metals are generally more reactive than plastics, making them more susceptible to damage during chemical activation.

The ultimate goal of the surface activation process is also an important consideration. For instance, chemical activation is better suited for applications where enhanced surface cleaning is the primary objective, while plasma treatment is the preferred option for improving surface adhesion.

Finally, a company’s budget is often the determining factor when selecting a surface activation method. For example, a high-tech UV treatment method may not be the cost-effective solution for a small-scale electroplating project. The key is to choose the most affordable method that will not compromise overall product quality.

Sharretts Plating Company: The Surface Preparation Experts

With more than 90 years of metal finishing experience, you can trust the experts at SPC to provide the most appropriate surface preparation method for your project and budgetary requirements. We offer a broad selection of surface treatment options, including complete ultrasonic washing and abrasive blasting services and much more. We also have extensive experience in plating on exotic materials such as plastics, ceramics and glass and in ceramic preparation for plating.

We Take a Consultative Approach to Surface Preparation

Our surface preparation services begin with an initial consultation. We will evaluate your project objectives to help you determine the right techniques for your unique manufacturing applications. We’re also willing to test a variety of methods to pinpoint the one that will produce the best results. We’ll even develop prototypes before moving forward with full-scale production. It’s all part of our unwavering commitment to achieving total satisfaction for every customer, regardless of project size or scope.

Contact SPC to learn more about our surface preparation services and to receive a no-obligation project quote today.