Zinc coating or plating is a process where a thin coating of zinc is applied to a substrate material, such as iron or steel, to protect it. The process of zinc coating is primarily used to protect metals from the effects of corrosion. There are a variety of zinc coatings that can protect iron and steel, and each has its own performance and characteristics.
The following are things to know about zinc coating and its implications.
1. How It Works
Zinc plating or coating is a way to protect from corrosion, as mentioned above. Zinc coatings can prevent the corrosion of the protected metal by forming an actual physical barrier. Even if the barrier is damaged, the zinc coating can still be protective.
The zinc and the iron or steel are joined, and then they’re put in an electrolyte, forming a cell. As a result, the zinc becomes the anode. The steel is what’s called a cathode. The zinc is what’s sacrificed so the steel doesn’t rust.
Zinc coatings can protect finished products, including structural steelwork for bridges and buildings, nuts, bolts, wire, and tubes. Zinc coating is used in the automotive industry as a cost-effective way to protect certain components like brake pipes and power steering. Zinc plating processes are used for heavily electrical transmission components, and it’s also used when manufacturing armored cars and tanks.
2. Types of Zinc Coating
In terms of protective coating, there are options, including organic and inorganic.
The two coatings have the same general purpose. The differences are primarily in the way they’re applied.
If you have iron or steel and you’re using inorganic zinc coating, it can be sprayed onto the material. Then, once you spray it, you add a topcoat to the inorganic zinc coating.
Organic zinc coating is an option if you don’t want to use an inorganic spray. The organic zinc coating can be applied by hand using a brush or roller. You might also choose to apply a topcoat over organic zinc.
While the most general categories of zinc coating are inorganic and organic, there are other classifications.
- Batch hot dip galvanizing: A prepared item is galvanized by being immersed in molten zinc. The surface is completely covered, so the coating is uniform. The galvanizing process’s primary benefit is the standard minimum coating thickness that can be applied no matter the person doing it. The molten zinc, as it’s applied in the galvanizing bath, will cover corners, edges, and seals, as well as rivets and seams. It penetrates any recesses so that it gives full protection to areas that, with other coating systems, could be at risk of corrosion.
- In-line galvanizing: Also known as continuous galvanizing, in this process, things like wire, pipe, and steel sheets might be protected. These allow for control of the thickness and characteristics of the coating. The products that undergo this process can often be further processed through roll forming or bending.
- Thermal spray: This is also known as metalizing. This is the process where semi-molten zin or alloys are sprayed onto fabricated items with wire or powder that’s flame-heated. This can provide thick zinc coatings, but thermal spraying can be more expensive than batch hot dip galvanizing.
- Electroplating: This is a versatile and economical way to apply a protective coating to small components made of steel. It’s often used for fasteners, especially ones with fine threads.
- Mechanical plating: This is an electroless method that’s used to deposit coatings of ductile materials using mechanical heat and energy. It can be used to plate steel parts with zinc, especially if they’re threaded.
3. Zinc Has Unique Anti-Corrosive Properties
Iron and steel are generally strong, but they have weaknesses. They corrode over time when exposed to oxygen, heat, humidity, and water. The best way to prevent this corrosion from occurring is to cover iron or steel. A zinc-rich coating is like a primer so that the zinc is what corrodes instead.
The concept, as was briefly mentioned above, is known as zinc being sacrificed instead of steel or iron. The corrosive product on zinc coating is zinc patina.
Eventually, zinc becomes ineffective because of corrosion, so the coating needs to be reapplied. The iron or steel underneath will stay in good shape as long as you keep applying the coating when it’s needed.
For anyone who wants zinc coating to last longer between needed applications, you can apply a thicker coat. The thicker the coat, the less likely it is that your zinc will corrode too much before you’re able to apply another layer of protection.
4. Zinc Coating Advantages
Zinc-coated steel has benefits, including:
- Low cost
- It’s recyclable
- High strength
These are in addition to the fact that it’s corrosion resistant.
Zinc is considered a unique element because its corrosion rate is significantly slower than other metals. Zinc can corrode at a rate that’s up to 100 times slower than steel or iron.
5. Environmental Performance
The following are some of the ways zinc can perform in different environments:
- Zinc is highly stable in environments that are warm and dry. The patina that forms during the initial exposure can stay intact, preventing any further reaction between the air and the galvanized coating.
- If there’s moisture in the atmosphere, a zinc oxide film can be turned into zinc hydroxide. Then, carbon dioxide that’s naturally present in the air will react, forming zinc carbonates. These compounds can then become stable and resist any further action, helping create longevity for the coating.
- In a rural area, galvanized coatings can often last for more than 100 years, but this can be shortened because of fertilizers and insecticides that are sprayed aerially.
- For areas near the coast, the rate of corrosion is more rapid due to sodium chlorides present in the atmosphere.
Finally, in industrial areas, because there are impurities in the atmosphere, like chemicals and sulfurous gases, soluble zinc salts are formed. These are removed by moisture, so the zinc is exposed to more attack. In extremely corrosive conditions of areas with high rates of industrial activity, it’s best to reinforce coatings with paint that reduces the impact of chemicals.