A few weeks ago, we published a brief introductory post about chrome plating. To satisfy reader demands, we have decided to follow up that post with a more detailed series of posts about the different steps involved in a typical chromium plating process.
This post, part 2 of the series, briefly describes the cleaning and surface preparation that precedes the actual plating process. Part 3 will cover finishing, and part 4 will cover the plating process itself.
Whether they are to be painted, powder coated, conversion coated, or electroplated, metal substrates need to be cleaned in order to promote maximum surface adhesion. Typical contaminants that may need to be removed include pigmented drawing compounds, unpigmented oils and grease, chips and cutting fluid, and polishing or buffing compounds.
Much attention is spent on a substrate’s cleaning process since it can have a huge impact on the quality and final appearance of a part or tool. There are many factors that need to be considered when designing a cleaning procedure for a given component, such as: the nature of the contaminants to be removed, the substrate metal type, the substrate’s geometry, and whether rust inhibition during the process is desirable – just to name a few.
There are numerous cleaning methods that may be utilized during difference steps of a cleaning sequence, each with its own set of pros and cons. A few examples include: solvent cleaning, emulsion cleaning, alkaline cleaning, acid cleaning, abrasive cleaning (such as bead blasting), and ultrasonic cleaning.
Some cleaners can leave residue on the to-be-plated parts, which will then have to be removed by one or more additional cleaning steps. This is not necessarily a drawback, but it adds to the complexity of the surface preparation procedure.
Surface Cleaning and Preparation for Electroplating
As mentioned, maximum plating adhesion requires that the substrate material be as clean and free of contaminants as possible. In addition to this, electroplating requires a completely active substrate surface. In other words, an oxidized but otherwise clean component is not yet ready for electroplating.
Following is an example cleaning sequence for a part that is to be electroplated.
Precleaning: This step may consist of one or more cleaning methods in order to remove most of the contaminants on a substrate’s surface.
Intermediate alkaline cleaning: This is done to remove any residual contaminants that were missed or deposited during the precleaning step. Alkaline cleaning is especially effective at removing polishing and buffing compounds, as well as oils that are difficult to remove during the precleaning step.
Alkaline cleaners usually contain a number of compounds – one ore more basic salts such as sodium hydroxide (lye); additives such as corrosion inhibitors, solvents, and chelation agents; and surfactants.
Electrocleaning: This is done to remove final traces of contaminants that are stubbornly adherent.
Acid treatment: This step removes any oxide film that may have formed during the cleaning sequence. Acid treatment also conditions the substrate surface by etching it.
Electrolytic cleaning: This process removes any undesirable films or coatings formed during the acid conditioning of the substrate. This is especially needed in the treatment and preparation of high-carbon steels, but may not be needed for low-carbon steels.
(A few words that many may be unfamiliar with are defined below. If you feel that additional definitions should be added, please feel free to contact us, anonymously or otherwise. Additionally, any feedback on the quality of this post or series of posts would be greatly appreciated.)
Chelating Agent: chemical designed to counteract the negative effect of metal ions.
Emulsion Cleaning: depends on emulsification where discrete contaminant particles are suspended in the cleaning fluid and separated from the substrate surface. Example of an emulsion – oil droplets in water.
Substrate: the base material, part, or tool that is to be coated.
Surfactant: surface acting agent that lowers the surface tension of a liquid. Dish soap is one type of surfactant.
This post was written with brevity in mind. Please feel free to ask questions or post a comment in response. Lastly, if you find any inaccuracies, please inform us; while we expend much effort in verifying the accuracy of our writings, we’re only human.
For additional information about the complete electroplating process, a good place to start is the ASM Metals Handbook Vol. 5: Surface Engineering. There may be a copy available via your local library, and if not you may be able to find one via your library’s interlibrary loan program, or perhaps the local college library.