By: Frank Caprio | On: January 4, 2017
So what is passivation, and how does it benefit my metal hoses and expansion joints?
One of the main reasons customers prefer using metal corrugated hoses and expansion joints in various applications is their resistance to corrosive chemicals and environments, especially at extreme temperatures. In most instances, these products are made from various grades of stainless steel unless the demands of the application require a special alloy. But what makes stainless steel “stainless”?
Stainless steel alloys contain various elements including iron, chromium and nickel. When stainless steel is first made, the chromium in the alloy reacts with oxygen in the air to form a layer of chromium oxide on the surface of the metal. This layer, while only a few molecules thick, effectively prevents water and other corrosives from attacking the free iron present in the alloy. It is therefore important to keep the surface of stainless steel very clean as it is made, otherwise this oxide layer may not form properly. Additionally, the oxide layer is able to quickly repair itself if scratched or buffed, thus maintaining its corrosion resistance over time.
However, if this barrier contains surface impurities or is worn away faster than it can repair itself, then the steel is no longer “stainless” and is vulnerable to rust. Keeping the chromium oxide layer intact ensures that stainless steel will resist corrosion. However, there are many factors that can reduce the effectiveness of the oxide layer, including:
- Highly corrosive chemicals, especially chlorides and sulfides
- Blemishes or contaminants on the metal surface
- Low oxygen environments
- Erosion corrosion, caused by abrasive or high velocity media
- Microbiologically Induced Corrosion (MIC) caused by microbial growth on the oxide layer
- Welding or other high-temperature conditions
- Cutting, grinding, or other machining
During processing and fabrication of stainless steel materials, it is possible that impurities may be introduced into the oxide layer, thus affecting its corrosion resistance. If the surface of stainless steel is buffed using a carbon steel brush, free iron from the brush can contaminate the oxide layer and cause ferric oxide (or rust) to occur on the surface of the stainless steel. This is called “rouging”, and may promote additional corrosion. Even using a stainless steel brush to buff a stainless steel surface can cause free iron contamination if that brush was previously used on a carbon steel part. That is why it is important for metal fabricators to segregate their grinding, buffing, and polishing equipment to avoid this cross-contamination.
Now that we know why stainless steel does not rust, let’s talk about what passivation is, and how it improves the corrosion resistance of fabricated stainless steel components.
Because stainless steel hoses and expansion joints are fabricated items, it is possible that the integrity of the chromium oxide layer may be compromised by various fabrication processes including machining, welding, buffing, etc. Although stainless steel “heals” itself by forming a new oxide layer when exposed to oxygen in the air, the integrity of this layer can be improved through a chemical process called “passivation”. Chemical companies may request that a metal hose or expansion joint be passivated after fabrication, especially if it is being installed in an application where added corrosion-resistance is desired.
Simply explained, passivation is achieved by placing a stainless steel part into an acid bath to dissolve any free iron on the surface of the alloy and to reform the oxide layer. Prior to passivation, the metal surface must first be thoroughly cleaned of any dirt, scale, or other contaminants. Then, it is immersed into the acid bath, which dissolves the free iron on the surface of the stainless steel without damaging the alloy underneath. The acid used is usually nitric acid with a concentration between 20% and 50%, at temperatures reaching 170 degrees F. Depending on the part being passivated, the immersion time may vary from 20 minutes to two hours. Nitric acid is a strong oxidizing acid, so it helps to form the oxide layer on the stainless steel. Citric acid may be used instead of nitric acid for passivation, but it does not help to create the oxide layer like nitric acid does, and therefore is not as widely used.
Passivation should be recommended in critical service applications where corrosive media is being transferred, as a means to ensure maximum corrosion resistance of the stainless steel. However, it is still important to select an alloy that is resistant to the media being conveyed. If your customer has any questions relating to alloy selection or corrosion resistance, let us know. We can consult various industry publications that provide corrosion-resistance data, in order to help your customer select the best alloy for their application. Passivation has an additional benefit beyond enhanced corrosion resistance; because the passivation process includes a very stringent cleaning stage, a passivated product also meets the industrial cleanliness specification for oxygen transfer. It is important that you specify when oxygen cleaning will be required on a passivated part so that proper documentation can be provided along with the component.
Passivation may help to ensure that our products last as long as possible once placed in service, thus providing maximum value. If you want to find out if passivation or any other option might be worth considering, contact us today with your application details and we will be glad to help.
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