Passivation is the process of treating or coating a metal so as to reduce the chemical reactivity of its surface. In stainless-steel, passivation means removing the free iron from the surface of the metal using an acid solution to stop rust. When the surface iron is removed, the other components of the alloy (primarily chromium, often nickel as well) are left behind as a surface layer over the underlying steel. Upon exposure to air, these elements react with oxygen to make an oxide layer that protects the rest of the steel from corrosion. This corrosion-resistant surface is often damaged through mechanical means or heat or chemical damage. When that happens, iron is exposed and therefore the item is yet again subject to rusting. For this reason, passivation may have to be performed on a regular basis. In both home and commercial settings exposure to things like cleaning solutions, bleach, or salt (oceanic environments) all will contribute to the necessity for normal passivation of the stainless-steel.
Historically nitric acid has been used to passivate stainless-steel, but recently a safer and simpler means using citric acid has been introduced. Unlike nitric acid, citric acid passivation has many advantages:
- Citric acid doesn’t remove other elements within the alloy effectively limiting the depth of the final chrome oxide layer
- Citric acid doesn’t introduce heavy metals (hazardous waste) into the bath
- Citric acid only removes iron
- Citric acid is far safer and environmentally friendly
Let’s take a glance at the most common methods and what they’re used for.
Anodizing is a way to passivate the surface of aluminium, titanium and magnesium parts. during this case, a coating or another material isn’t added on top. Rather, the particular surface chemistry of the metal is modified or “converted”, so this is often called a conversion coating.
The top layer of metal – about 5 microns thick – is first cleaned and stripped during a bath of sulfuric or hydrochloric acid. Then, the part is placed into another tank of liquid, called the electrolyte. When the target piece is given a positive electrical charge (the “anode” in anodizing), it’ll attract the negatively charged ions floating around within the electrolyte. They then bond with the surface, creating a new, strong oxide layer.
Why do you want this? Once you have got aluminium oxide on the surface it’s very stable and scratch-resistant. It protects against rust or corrosion, and also makes an excellent primer for extra treatment like paint or powder coating. When dyes are added to the electrolyte, they create the customary coloured look that we associate with anodized parts.
Bluing is another conversion coating, but this works on steel. Like with anodizing, the target part is cleaned with acid then dipped into a different electrolyte chemical solution which forms the familiar blue surface that’s often found on gun barrels. A blued surface looks nice and it helps to reduce glare but it’s very thin, so regular treatments of oil are needed to keep up corrosion protection.
Parkerizing is comparable but more robust than bluing. The surface comes out looking a dull matte grey, so this is often used where the cosmetic finish isn’t important but better environmental performance is. Dark grey or black oxide conversion coatings using this same technique are sometimes finished fairly inexpensive treatment of mass-produced automotive parts.
Although named after the Italian scientist Luigi Galvani, the method comes from India originally and refers to dipping steel or iron parts into molten zinc. this is often 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 isn’t especially precise and doesn’t aid in mechanical performance.
The zinc also has the secondary advantage of being a sacrificial anode. Let’s say the outer layer of zinc is broken, exposing the steel underneath. Time for free oxygen to attack the steel and create rust, yes? Not so fast. The oxygen radicals have a better time bonding with the zinc and making zinc oxide instead, therefore the zinc “sacrifices” itself to preserve the steel, a minimum of for a little while.
Another variant here is yellow zinc plating, which is extremely familiar due to the typical iridescent yellow-green color that’s so common on nuts and bolts and other low-cost hardware.
A common plating process that creates the shiny, polished surface that you just find on everything from kitchen faucets to hot rod engines. And it’s not only for metal; many plastics are often chrome-plated also, so long as they’re not damaged by the chemical bath. Chrome is gorgeous, hard, and sturdy, but there are some trade-offs.
Chrome plating uses chromium and therefore the process may be environmentally harmful so it must be carefully controlled – that’s one among the reasons for the higher cost. And if the chrome starts to flake or peel off of a surface you can’t just “touch it up” like with paint – the complete 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 may be polished to a high luster if desired. Like chrome, nickel is often used for an ornamental finish, for corrosion protection, and to increase surface hardness and abrasion resistance.
Nickel is additionally used as a base coat for a later application of chromium. the utilization of nickel as a plating material isn’t considered as hazardous as that of chromium, though it should be more expensive in many applications.