Nothing is more satisfying than a shiny new car...
Or is there? Chrome goes beyond mirror-like bumpers on cars. To get the chrome onto automotive parts and more, you have to put in a bit of work plating. With the amount of work that goes into plating, how do you help determine that the chrome finish will come out every time? Testing the plating solution helps to get the part right every time.
What Is Chromium?
While you are most likely familiar with chromium on the bumper of your car, did you know that it is actually an element that you can find on the periodic table? That's right, you have an element that you see every day. It is a naturally occurring metal.
What Is Chromium Used For?
Chromium is frequently used as a finishing layer on metal pieces (car parts, aerospace parts, decorations, electrical, and more). You may be thinking, why plate chromium? Isn’t that just being cheap? Well, yes and no. Chrome is heavy, and can be expensive. To make sure the metal parts aren’t cost prohibitive, and are functional, objects are plated with chrome. Chrome plating is used in a variety of industries and on a myriad of products. However, chrome is used for plating for two main reasons. The first is for decoration, and the second is for function.
Everyone loves shiny and pretty things, and chrome is perfect for giving a mirror-like finish. Decorative chrome is a very bright and reflective metal that gives a desirable finish to metal pieces. Even though it is purely for decoration, decorative chrome does do a pretty good job at resisting corrosion. If you have had something plated with chrome, and the chrome flaked or had other similar issues, the problem is actually with that particular plater's process, not the metal itself.
Decorative chrome is also called show chrome or commercial chrome. Decorative chrome is plated incredibly thin, at 1/1,000,000th of an inch1, or 0.05 - 0.5 micrometers thick2. Even with the super thin plating, decorative chrome usually consists of at least two layers. In between each layer, the platers highly buff the metal to get it as smooth as possible. Decorative chrome can be plated onto steel, brass, aluminum, copper, plastic, nickel, and stainless steel3. Nickel can be used as a "strike" layer to get the chrome to bond better to the object getting plated. There should be two layers of nickel plated onto the part prior to plating the chrome (if nickel is being used).
Hard chrome is used for function. Just as the name suggests, hard chrome is tougher than decorative chrome. It provides wear resistance, friction reduction, improved durability, and oxidation resistance to the object. You may ask, is this a different type of chrome all together? Nope! Hard chrome is simply thicker layers/deposits of normal chrome. The additional strength also comes from what the metal that is being plated (i.e. the strength of the steel). Hard chrome is also called industrial chrome and engineering chrome. The look of hard chrome will differ from decorative chrome as it may have a more brushed appearance, but it can be buffed to a high shine.
Is Chrome Dangerous?
Chromium should be taken seriously, and safety procedures should always be closely followed when working in chromium plating. If you are not in the industry, and were hoping to do some chrome plating for home projects or as a fun science experiment, we would recommend that you do not. Chromium baths are heavily regulated due to the toxic nature of the bath components (both the acids, as well as the metals present). Chromic acid is a primary component in chrome plating baths, and it is a known carcinogen if absorbed into the body (skin contact, fume exposure, ingestion).
How Do You Plate Chrome?
Chromium plating all boils down to electroplating, also known as electrochemical plating. You always start with a base metal, or a strike layer on the piece. Electroplating nickel first is very common.
Plating chrome onto an object follows the typical rules of electroplating. You need to initiate a redox (oxidation-reduction) reaction in your plating bath. For this to happen, the plating bath is made up of 4 primary parts: the cathode, the sacrificial anode, the electrolyte/plating solution, and a power source/rectifier. Yes, that is simplifying the process, but those are the essentials. All of these components complete an electrical circuit.
Every plating shop and plating line will have a variation on the basic chromium plating procedure. Between each step they will most likely clean the metal piece. Many shops will use a nickel strike on the base metal as an in between layer before plating the chromium. The end stage is always inspecting the piece and buffing it to the desired finish.
Chrome Can Be Used To Refinish A Piece
When a metal object needs to be repaired or refinished, chrome is an excellent choice. The steps for refinishing a piece with chrome are below! Keep in mind, this is just an example.
- Strip it down. Previous plating flaking, flaws all have to go. Luckily, this can be done using a variety of methods including pickling and sandblasting. This step also helps to remove contaminants from the surface of the piece.
- Fix any imperfections and damage. Pitting and cracks should be filled and mended.
- Shine it up. Polish the piece until the surface layer of metal is removed.
- Rub-a-dub, time for the tub. Cleaning the pieces to be plated is essential. After the previous steps, particulates, oils, finger prints, metal shavings and more can accumulate on the surface. All of these contaminants can be catastrophic to the chrome plating properly. Cleaning the pieces can involve surfactants, RO water, pickling, and acids.
- Plate copper onto the piece.
- Clean the piece again!
- Plate nickel onto the piece. (This step will be repeated to have at least two layers).
- Clean again (squeaky clean here folks).
- Plate chrome onto the piece. (Just like the nickel, it can be repeated to have multiple layers.)
- Clean the piece.
- Buff it to a shine/desired finish.
- Check the finished piece and ensure it is up to spec.
Why Is Testing Chromium Important?
Chromium should be tested in the plating baths themselves and in the waste stream. This is due to wanting to keep the bath balanced, and to comply with regulations. Bath components can include hexavalent chrome (Cr6+, chromium trioxide), and concentrated sulfuric acid (this keeps the pH close to 0). Baths with those components tend to give finished pieces higher corrosion resistance. This formulation is less expensive than tri-chrome. A major downside to this formulation is the toxicity, and the poor throwing power (coverage).
Another formulation is known as trivalent chromium, tri-chrome, or chrome-3 plating. It works similarly to Cr6+ plating, but the bath makeup is different. Baths with this formulation uses Cr3+ in forms such as chromium sulfate or chromium chloride. The anodes in the plating bath have to be shielded to prevent Cr3+ oxidation. This bath formulation is far less toxic than Cr6+, allows you to plate more efficiently, and offers better coverage. Disadvantages to this plating bath formulation include needing to use more expensive chemicals, and it provides lower corrosion resistance when you are looking to plate a hard chrome layer.
How Do You Test For Chromium?
Titration is a common method for determining the amount of chromium available in plating baths, as well as in the wastewater stream. Using titration, you can determine the total amount of chromium as well as chromium VI. Chromium III can be determined via a calculation where you plug in the results from the titration for total chromium and chromium VI. You simply need an ORP electrode, an automatic titrator, titrant, and a few reagents and you are good to go!
For more information regarding how Hanna Instruments can help you with your plating needs, contact us, at firstname.lastname@example.org or 1-800-426-6287.
Allison graduated from Bryant University with a Master’s Degree in Global Environmental Studies. She is passionate about nature, and how science is connected to the world around us. At Hanna, she provides an array of content and support to customers through the Hanna Blog, SOPs, and Data Sets.
Allison may be reached at email@example.com.