By: Abby Svitana | On: March 25, 2020
So why do they call it that?
Stainless steel products are all around us: steak knives, appliance fixtures, cookware, and (most importantly) metal hose and expansion joints. We see stainless steel items so frequently that we don’t really give the term much thought. But, what does it actually mean to refer to a steel as “stainless?”
The answer lies in the chemistry. Stainless steel alloys contain various elements such as iron, chromium and nickel. When stainless steel is first made, the chromium reacts with oxygen in the air, forming a layer of chromium oxide on the surface of the metal. While this chromium oxide layer is only a few molecules thick, it serves a big purpose. It protects the integrity of the alloy, preventing water and corrosives from attacking the free iron present in the alloy. Even if the oxide layer is scratched it will quickly repair itself and maintain its corrosion resistance. It is this resistance to corrosion and rust that give these alloys their “stainless” quality.
Why choose a stainless steel?
Because of its “stainless” quality, stainless steel provides a valued alternative to non-metal hoses in certain environments and applications. For example, due to its outer oxide layer stainless steel is able to maintain its integrity in moisture-laden or corrosive environments without rusting. It also provides better resistance in extreme temperatures than non-metal hose materials. Certain alloys of steel, such as 304 and 316 are rated to withstand temperatures as low as -425 degrees Fahrenheit up to around 1300 degrees Fahrenheit. Other materials, such as rubber or PTFE will crack at low temperatures and melt at such high temperatures.
Keeping your steel “stainless”
While stainless steels offer great corrosion and rust resistance naturally, there are certain conditions that can lessen their outer oxide layer and reduce their corrosion resistance. For example:
- The presence of highly-corrosive chemicals, such as chlorides and sulfides
- Contaminants on the surface of the metal
- Low-oxygen environments
- Erosion corrosion, caused by abrasive or high-velocity media
- Microbiologically Induced Corrosion (MIC)
- Welding or other high-temperature conditions
- Cutting, grinding, or other machining
If an assembly is likely to experience these conditions, an additional treatment can be applied to help preserve the chromium oxide layer and provide additional corrosion resistance. This treatment is commonly referred to as “passivation.” Passivation is done by placing stainless steels into a bath of 20-50% nitric acid solution at temperatures up to 170 degrees Fahrenheit. Depending on the part being passivized, the immersion time can vary between 20 minutes to two hours. This nitric acid bath dissolves any free iron on the internal and external surfaces of the steel without damaging the underlying alloy. Once removed from the nitric acid bath and exposed to the air, the oxide layer on the surface of the steel quickly rebuilds itself, strengthening the steel’s corrosion resistance.
Along with proper alloy selection, passivation can provide valuable additional corrosion resistance and help an assembly achieve the longest possible service life, especially in regards to critical service applications where corrosive media is being transferred. But remember- just because a steel is stain-LESS does not mean that is stain-PROOF. If you have concerns regarding corrosives in your applications or think your application may be better served by utilizing passivation for your stainless steel assemblies, contact our inside sales team. With access to industry resources on corrosion-resistant data and combined application expertise, we can recommend and design an optimal solution for any application.
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