Let’s look at the most common passivation techniques so you can make an informed decision about which one is best for your next project.
Anodizing is usually done to aluminum titanium and magnesium. This is not plating, because you’re not adding some other material to the surface. Rather, it’s a way to convert one chemical compound to another, so it’s called a conversion coating. Your products may be exposed to oxygen, either from the atmosphere or in liquid water. And oxygen is highly reactive. When oxygen contacts the surface of the part, it has a tendency to create a chemical reaction which can lead to corrosion or rust.
To combat this, it’s possible to passivate the surface. Passivate means to subdue – to “pacify” – the tendency of the surface chemistry to overreact. So the part in question may be dunked into a cleaning tank of sulfuric or hydrochloric acid, which etches away the surface layer.
Inside the bath of electrolyte an electrical current is applied. The target piece – the thing that you want to be anodized – is given a positive (+) charge. This is called the “anode”, thus you have anodizing.
Negatively charged (-) electrons in the electrolyte will be attracted to the positive surface of the target piece. There, the oxygen forms an oxide layer on the target, like Aluminum Oxide for example.
A thin layer of Al2O3 will passivate the surface and inhibit further corrosion. Additional colored dyes can be added during the anodizing process to create the matte-finish metallic look that we’re all familiar with. Typically this oxidized layer is between 5μ to 150μ thick. In addition to corrosion protection, the surface is made harder and forms an excellent base for paint or powder coating.
Passivation is an electrochemical method for altering the surface properties of metals and some plastics. This is done to improve hardness, corrosion protection, paint adhesion and other qualities.This is also a conversion coating, so there’s a similar process involved as with anodizing. It’s often used on steel parts for firearms, providing a degree of corrosion protection while enhancing cosmetic appearance and reducing glare.
Te target metal is dipped into a hot bath, typically of potassium nitrate and sodium hydroxide, which reacts with the steel to form magnetite, Fe304. The surface film is very thin and needs to be treated with oil to retain corrosion protection.
A related process for firearms is parkerizing. More robust than bluing, it’s a phosphate conversion using a zinc or manganese electrolyte, and which produces a matte grey surface finish.
In addition, black oxide coatings, of which bluing is one sub-type, are common for mild steels used in the automotive industry when surface quality is secondary to the economy of mass production
Although named after the Italian scientist Luigi Galvani, the process comes from India originally and refers to dipping steel or iron parts into molten zinc. This is called “hot-dip” galvanizing, and it’s immediately recognizable to any school kid climbing a backyard fence. The familiar flat grey, rough finish of zinc provides corrosion protection on mild steel but the finish is not especially precise and doesn’t aid in mechanical performance.
The zinc also has the secondary benefit of being a sacrificial anode. Let’s say the outer layer of zinc is damaged, exposing the steel underneath. Time for free oxygen to attack the steel and create rust. Not so fast. The oxygen radicals have an easier time bonding with the zinc and making zinc oxide instead, so the zinc “sacrifices” itself to preserve the steel, at least for a little while.
Another variant here is yellow zinc plating, which is very familiar because of the typical iridescent yellow-green color. This is a low-cost option for mass produced parts that need corrosion protection without a high-end finish.
A common plating process that makes the shiny, polished surface that you find on everything from kitchen faucets to hot rod engines. Chrome is beautiful, hard and durable, but there are some trade-offs.
Chrome plating uses chromium and the industrial process can be environmentally harmful so it must be carefully controlled – that’s one of the reasons for the higher expense. And if the chrome starts to flake or peel off of a surface you can’t just “touch it up” like with paint – the entire piece must be stripped and re-plated from scratch.
Hard chroming is another technique used to put a super-durable coating on moving parts like bearings and shafts.
One possible substitute for chrome is nickel plating, which can be polished to a high luster if desired. Like chrome, nickel can be used for a decorative finish, for corrosion protection, and to increase surface hardness and abrasion resistance.
Nickel is also used as a base coat for a later application of chromium. The use of nickel as a plating material is not considered as hazardous as that of chromium, though it may be more expensive in many applications.
Which technique is best for you?
These are the most common passivation techniques we use every day, and each has their inherent advantages, costs and limitations. If you’re still not sure which one is right for you read more about our finishing services or talk to one of our sales support engineers today for more information and a free quote.