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How to prevent corrosion on your gears

From Stephane Baleon, posted 08/01/2020

gear splash in water

Corrosion is a natural process that might affect your mechanisms and decrease its operating life. It is important to take this oxidation in consideration when designing your mechanism, choosing your components and storing them.

Prior to seeing what can be done to prevent corrosion, it is important to understand the different types of corrosion that can occur on your mechanism.

- Oxydation, mainly rusting : Iron atoms from steel alloys is reacting with Oxygen in the presence of water

- Galvanic corrosion: two dissimilar metals and an electrical contact through an electrolytic media

- Other variations linked to the two above. For example stress corrosion cracking (temperature and mechanical stress) or fretting (micro-oscillations between 2 materials).

This article focuses on single element steel allow protection, for exemple spur gears, racks, bevel gears or even internal gears. So galvanic corrosion, fretting or corrosion cracking won't be covered here. Only rust.

Proper packaging for storage

Your mechanism might be operating in difficult conditions. However corrosion protection starts prior to having gears operating in their environment. It all starts with storage. Because you might keep gears and components for few years before they even get integrated!

If you ever received gears from Adamant Precision, you saw that they are in a sealed package containing oil. The oil covering the whole surface of the components prevents contact between iron and oxygen leading to corrosion. This packaging allows you to keep the gears for years with no corrosion at all.

custom gear packaging

custom gear packaging 2

 

Most of the time, this is sufficient. But for additional protection, it is possible to replace the air in the packaging by an inert gas, like Nitrogen.

If oil or grease coating on the gears is not possible due to cleanliness requirements, another option is to use anti rust paper and/or VCI packaging. Anti rust paper is designed to absorb humidity. The VCI packaging is impregnated with corrosion inhibitor agents that will be slowed released in the air protecting the components on a short term basis.

If you have a specific packaging requirement for your gears, please get in touch and Adamant Precision will provide the best solution for your application.

Failure to protect your gears and components during storage might reserve you some bad surprises, as seen on the pictures below.

corroded gears

Cleaning

Making sure that no corrosion agent are in direct contact with your gears and components material is important. 

The first very important step is to clean and passivate the gears. Even if using a strainless steel alloy, during manufacturing, due to the tools used to cut the material and cutting fluid, some iron particles might be stuck on the surface of your gear. These can easily corrode and transform to iron oxide (rust). 

To prevent this, an effective way to clean the components is required. ASTM A380 describes standard practices for cleaning, descaling and passivation of stainless steel parts.

Cleaning can be done using some of the following methods:

- Vapor degreasing
- Ultrasonics
- Steam
- High pressure water jetting
- Using alkaline, solvent or detergent cleaner

gear splash in water

Passivation

Once cleaned, passivation process creates a thin layer of very inert material on top of the original material. This layer won't react with corrosion agents.

For stainless steel passivation, the use of an acid will first remove any embedded iron from the metal. Then the raw material left behind will naturally react with the air oxygen and form a oxide layer protecting the underlining steel material.

For steel, the use of Nitric acid or Citric acid is possible, but the latest is more environment friendly.

One very important note about passivation through: one designer must be very careful when specifying passivation on its gears, because if the part has narrow features, difficult to clean, then acid will not be fully removed and the part might fail sooner than expected!

Coatings

If passivation is not possible, or for improved corrosion resistance, it is possible to coat the gears or components. Usually using a coating will serve a double purpose by improving as well some other design aspects so there are many possibilities.

- DLC (Diamond Like Carbon) coating for very low friction and high hardness

- Gold for high electrical conductivity

- Nickel or Chrome mostly for corrosion resistance

- PTFE non reacting to most chemicals

- Polymers for electrical insulation

- paint for higher thickness

- ...and many others!

Conclusion

Many ways to fight against corrosion were presented in this article. Keep in mind that this is only applicable to a single component, but there are additional tricks and design rules to follow when designing a full mechanism,particularly with various materials: selecting acceptable material pair potential, insulating parts with different materials, using sacrificial anodes...

Regarding the gears, Adamant Precision delivers everything in protected oil filled packagings. We can also work with your own specifications: special anti-rust paper / no oil, VCI film, cleaning and passivation or coating. If you have specific requirements please get in touch.

If this article was helpful, please feel free to share it with your fellow Engineers. 

Thank you!

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