The relationship of plating with noble and base metals
I’m sure you all are familiar with the term, “noble metals.” Well, do you know about “base metals?”
“Maybe noble metals are expensive and base metals are cheap?”
Actually, in this context metals are not classified by their worth! The keyword here is “ionization,” which has a very strong connection with plating. Here, I’ll explain each term:
Noble metals：These metals are stable, show little aging degradation*, and do not easily rust. They are not easily ionized.
Base metals：These metals rust easily on their own. They are easily ionized.
* Aging degradation: A state where the physical properties of a substance change with the passage of time
“Ionization tendency” is an indicator used to express the degree of “ionization.” Let me explain in a little more detail.
“Ionization tendency” expresses how easily oxidation (ionization) occurs in a fluid. In other words,
The smaller it is…the harder it is for a metal to ionize, oxidize, corrode, and rust
The larger it is…the easier it is for a metal to ionize, oxidize, corrode, and rust
We call metals that do not ionize easily “noble metals,” while metals that do ionize easily are called “base metals.”
Some examples of noble metals are gold, platinum, silver, and copper. Now you understand why they are called “noble metals!” Even so, it seems a bit cruel to call something a “base metal.” After all, it’s not that these metals are inferior…
Which elements can be used in plating?
The “periodic table of the elements” is a chart that lists elements by their atomic numbers. Let’s use this table to classify the elements that can be formed into coats through plating. Let’s say that these elements “play friendly” with plating. In order to classify which elements can be used in plating, we first assume:
Elements or combinations of elements that include metals
Then, as I mentioned earlier, we can split elements into:
1.Elements that can be used for electrolytic plating
2.Elements that can be used for electroless plating
3.Elements that can be used for plating with alloys
As you can see in the video, there are a surprising number of elements that “play friendly” with plating.
Introducing the elements that “play friendly” with plating
Let’s stop studying for now so we can meet some elements that “play friendly” with plating!
- Most stable and attractive: Gold-plating
- When you think of noble metals, maybe the first one that comes to mind in gold. As I mentioned at the beginning of this episode, gold is the most resistant to ionization, and is stable in the atmosphere. It also represents “universal value” to most people.
Gold-plating plays a role in a wide range of applications. This useful substance is often used in electronics and advanced terminals/connectors.
Roles: Decoration, anti-rust, electrical properties, solderability
- Mirror-finishing “all-star” that is also used for its antibacterial effect: Silver-plating
- Silver is not only pleasing to the eye, but provides an antibacterial effect. It is therefore used on things that we often touch, such as tableware.
It also offers low electric resistance, so silver-plating is often used on connectors, electrical contacts, and other parts. Silver-plating technology is still in use today in manufacturing mirrors, and is most often used in applications involving light reflection.
Roles: Decoration, antibacterial properties, electrical properties, optical properties
- Often seen in electrical applications: Copper-plating
- Copper is highly conductive (that is, electric current flows easily through it), so it is often used in applications related to electrical wiring, such as printed circuit boards. Copper is inexpensive compared with other noble metals and copper-plating is relatively simple. It is therefore often used for base plating or in place of gold-plating.
One other characteristic of copper is its excellent thermal conductivity, which is why it is also used in cookware.
Roles: Electrical properties, heat properties, abradability
- The longest-serving plating method, used since antiquity: Tin-plating
- Tin-plating has been used since long ago to prevent rust. It is still often used today for such applications as electronic component connections.
It is also often used in foodstuff-related applications (such as processing the insides of canned goods), since it has little effect on the human body.
Roles: Anti-rust, decoration, solderability
- The most versatile plating method: Nickel-plating
- Nickel is an alloy with a variety of characteristics. You could think of it as the “all-rounder” of the plating world.
Nickel-plating is used for a variety of applications, such as hardening surfaces, creating surfaces that are flat with little resistance, coping with the magnetic force of electronics, and improving the look of a surface.
Roles: Precision properties, hardness, abrasion resistance, optical properties, heat resistance properties, magnetic properties, anti-rust
- Superior durability: Chrome-plating
- Chrome-plating is often used to prevent rust or for decorative purposes. Another option is trivalent chrome-plating, which—in contrast with the image most people have of chrome-plating—has less of a burden on the environment. It is used on hard surfaces, and it often used on such parts as shafts, valves, piston rings, and axle bearings. Chrome-plating is also used for molds, due to its non-adhesiveness.
Roles: Anti-rust, decoration, hardness, abrasion resistance
- Appealing low cost: Zinc-plating
- Zinc-plating has been used as anti-rust plating for a long time. One common example is the hot dipping method known as “hot-dip galvanization.” As a low-cost means to prevent rust on iron, it is still used in large structures, bolts, nuts, aerial wiring fittings, and other parts.
Roles: Anti-rust and decoration
- Working together with substances that offer unique abilities: Composite-plating
- Composite-plating is a kind of plating that demonstrates a variety of characteristics when combined with a non-metallic substance in a eutectoid alloy. For example, combining PTFE (polytetrafluoroethylene, a resin formed from fluorine and carbon) and nickel in a eutectoid alloy can greatly decrease the surface friction of the resultant substance, and can be used in applications that require water repellency or smooth machine component movement.
Roles: Lubrication properties, water repellency, mold releasability
Today, I introduced only the major elements that “play friendly” with plating. Actually, there are many more! Plating offers a variety of characteristics depending on the metal being used, and is an active part of our lives.
Having said that, this doesn’t mean that we can use all of the many applications for plating. A lot of these are likely still in development. Combinations of metals and where they are used could very well open up a brand new world for us.
We will continue to see the creation of new technologies in the world of plating.