How to Control Corrosion: Material Selection for Rings and Springs

Welcome back to the Ask the Expert blog. The blog is designed to help educate you on all things retaining rings and wave springs. This week, we talk about what corrosion is and why it’s an important factor to consider when choosing the appropriate material for retaining ring and wave spring design.

What is Corrosion and Why Does it Matter for my Application?

Corrosion is a naturally occurring process that causes gradual degradation of a material. Corrosion can be categorized into general and localized corrosion. General corrosion results in uniform thinning of material, while localized corrosion results in pitting.

Corrosion can cause a variety of problems within your application, including contamination due to flaking, discoloration, and hydrogen embrittlement, because of the chemical changes taking place.

Corrosion can be a controllable process given environmental conditions and appropriate material selection. This is where a Smalley engineer is critical helping you choose a material for your application.

What Material Should I Use for a Ring or Spring?

Carbon Steel and Finishes

Carbon steel is a ferrous alloy that is used as a standard material for spiral retaining rings and wave springs. It is best suited in applications set in a protected environment, as it can corrode if there is no rust inhibitor applied. An industry example is a retaining ring in a transmission where it is submerged in transmission fluid. 

While carbon steel is more economical in comparison to stainless steel, it is not inherently corrosion-resistant. Upon exposure to oxygen, the iron surface undergoes oxidation to form iron oxide, better known as rust. This layer of rust flakes away, which exposes more free iron and the corrosion cycle continues.

The animation below depicts the corrosion process:

Corrosion resistance can be achieved by minimizing exposure to oxygen. This is done by applying a finish to the carbon steel surface. As part of our standard manufacturing process, Smalley applies a rust inhibitor on all carbon steel parts. As an alternative, two other finish options are zinc plating and zinc phosphate coating. However, achieving full coverage of plating or coating between the turns of a spiral retaining ring poses a challenge. While carbon steel may be less expensive in large quantities, stainless steel may be more cost-effective in smaller quantities.

Stainless Steel and Passivation

Stainless steel is a corrosion-resistant material that has a minimum composition of 12% chromium. Different grades of stainless steel have different corrosion resistance capabilities. Smalley engineers typically use stainless steels from the 300 series as well as precipitation-hardened 17-7. The grades are based on the alloying elements, as well as their respective compositions. The table below outlines use-cases for the different types of stainless steel.

Table 1. Stainless Steel Types. Types of stainless steel and their respective applications.

Stainless steel has a natural oxide layer due to the chromium composition. The presence of the passive layer blocks the formation of iron oxide, which makes it resistant to corrosion. Oxygen is required to create a passive layer that makes stainless steel, “stainless”.

Fig 1. Stainless Steel Passive Layer. Chromium reacts with oxygen to form a protective barrier on the surface.

Although stainless steel is corrosion-resistant, it is not corrosion-proof. Free iron on the surface can inhibit the formation of iron oxide. Parts can be passivated to remove free iron to ensure optimum corrosion resistance. The most common passivation and Smalley’s standard is AMS 2700, Method 1, Type 2, Class 3. Please consult a Smalley Engineer for other available methods.

Exotic Alloys

Smalley also offers exotic alloys, including NACE compliant materials such as Inconel^® and Elgiloy^®. These are used in cases where stainless steel is unable to withstand certain extreme environments. More information on our exotic alloys may be found here.

Summary

There are a variety of key factors when it comes to choosing the right material for your application including environment, budget, and cycle life. Below is a summary table outlining the corrosion resistance potential for some of the materials that we offer. Always consult with a Smalley engineer to choose the best possible material for your application.

Table 2. Comparison of Materials to Environment. Materials and their designated environments.

That's all for this week. Check Ask the Expert next week to learn more about wave springs and retaining rings.

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