Tuesday, July 6, 2021

Types of Corrosion-Resistant Coating

Types of Corrosion-Resistant Coating

Today, many industries choose to work with lightweight metals and use them as parts of their machinery because they have versatility. These metals like magnesium, titanium, and aluminum have played essential roles in aerospace, automotive, and consumer workplaces. Their combination of exceptional strength and abundance is just two of the reasons why engineers prefer them all over the world.

Some of the alloys may competently offer corrosion resistance, even if a coating does not treat them. However, the surface treatments are inevitable in the finished product if the owners want longevity and maximum performance. A corrosion-resistant coating is needed to prevent attacks from molten salts and other chemicals. These methods will also prevent the process of galvanic corrosion where two different metals come in contact with each other, and the less “noble” one will suffer accelerated wearing away.

For example, magnesium is well-known for its corrosion resistance. However, others may not be aware that the 7xxx, 2xxx, and other aluminum alloys related to the high-strength families are susceptible to corrosion and resistance.

Selecting suitable methods is very important, and they are usually incorporated with the manufacture and component design. Some of the ways have unique advantages, but they can pose potential issues as well. Others are combining the coating and treatments that will help the metals last longer. Some of the methods that you may need may be the following:

1. Anodizing

One of the more popular methods of improving the resistance of aluminum against wearing is anodization. It can involve a lot of steps in the corrosion-resistant coats when it comes to this metal. Read more about anodizing on this site here.

The first few stages may involve immersion inside a bath where the solution is conductive. This bath usually contains a low pH acid, and it connects the alloys of the anode to the electrical circuit. When there’s a current that’s applied, an oxidation reaction will occur.

The natural oxide will be thicker, which will serve as the first outer layer of the aluminum oxide. Some of the thicknesses are usually altered by an extension of the coating time. This is offering various application ranges:

  • When the coating is applied lately, this can be a good initial treatment for paint and any other subsequent coating that will be used in the future. 
  • Specific color effects are achieved, especially if there’s dye involved
  • Thin layers like the ones which are less than 20μm preserve the aesthetic of the metal type, and it’s translucent in nature.

The decision involving the coating will play a significant role in determining the resistance in conditions where they are used outdoors. The under-intensive indoor stress can be a minimum of 20μm, and this is recommended. Layers of at least 10μm are recommended for extra thickness. The higher voltage addition can crack the protective oxide layers and make the metal more porous.

The cracks often occur on the corners of the metal, limiting the edge and protection offered by the anodizing layers. The hot water seals are also used for more substantial protection, but a more effective one is utilized to help solutions like sodium dichromate or nickel acetate.

2. Plasma Electrolytic Oxidation or PEO

Types of Corrosion-Resistant Coating
PEO involves the utilization of plasma discharges to help change the surfaces of light metals. The qualities are both dense and complex. Learn more about PEO here: https://pubmed.ncbi.nlm.nih.gov/34067483/.

The components of the metals are put into a bath with electrical charges. The current will be used to grow the layers uniformly on the surface. The PEO can occur in three stages which are the following:

  1. The substrate’s oxidation as what is seen on the anodizing process 
  2. Start of the co-deposition of the electrolytes in the coatings 
  3. Modifications of the resulting layer through plasma discharges

The wear-resistant coats from the PEO are also complex, and they are made for lightweights including magnesium, titanium, and aluminum. When comparing the PEO to the anodized coating, this has a higher chemical passivity and hardness that is very advantageous towards the pore structures. It creates stronger adhesion and high strain tolerances.

Aside from the chemical and physical characteristics, this process is considered to be eco-friendly. This is because the benign electrolytes are available to be used later on, and the by-products of the oxidation process are usually non-toxic. The electrolytes are free from heavy metals, ammonia, acids, and chromium. While the lower concentrations of the alkaline solutions can be easily disposed of, they are common hazards.

3. Chromate Conversion Coats

One of the best and most effective methods is called chromate conversion coating. However, the increased scrutiny of the government and regulatory manufacturing processes has led to the gradual phasing of this corrosion resistance technique.

This is used to clean the metallic surfaces as well as the various additives. The chemistries can differ in a wide range, but techniques may include potassium chromate, sodium, chromic acid, and dichromate solutions. The utilization of the additives has risen to cause the redox reactions while leaving the passive film containing IV oxide and the substrate metal’s hydrated compounds. This is providing a higher corrosion resistance overall.

Higher protection will stem from the ability of the chromium compound to reform the oxide film in a damaged area to the coating that was exposed to the oxygen in the atmosphere. This is often referred to as a self-healing process, and this uses a similar mechanism in the creation of stainless steel.