You Could Be Using the Wrong Corrosion Chart

By: Frank Caprio | On: September 18, 2018

Corrosion is the enemy. One of the many features of metal hoses and expansion joints (EJs) is their ability to resist corrosive media or environments. It is therefore critical that cost effective materials are selected which are resistant to both the media being conveyed and the environment in which the item is installed. We discussed some of the available alloys in our July blog post. Let’s take a look at this material selection process and how using the correct resources (like corrosion charts) can dramatically affect performance and safety.

Go to the source

Many metals producers offer technical data sheets for the various alloys they offer. These data sheets provide great information regarding the composition of the alloy, as well as its physical and mechanical properties. These sheets also do a good job of providing general statements regarding the suitability of the alloy for use in different chemical environments. Sometimes, these overviews are sufficient to verify that a certain alloy will work as expected in a given application. However, there are times when more detailed information may be required, especially if the alloy will be exposed to potentially corrosive media in critical applications. In these cases, more specific testing and evaluation resources should be consulted.

There’s an app for that (or a catalog, or a webpage…)

Many fitting manufacturers offer corrosion resistance information in their literature. We’ve all seen these at some point: the charts in the back of their catalog listing the various alloys of products they offer, and dozens of chemical names running down the page. Each chemical has a corrosion resistance rating for each fitting alloy, with the compatibility rating usually designated by a letter or a number. Here’s my cursory yet colorful stab at what a typical fitting cross reference might look like:

While these charts are okay to use for fittings, we recommend that our customers DO NOT USE THEM to determine the chemical resistance of a metal hose or expansion joint. It is important to understand that metal hoses and expansion joints are flexible because they incorporate corrugations which are formed from relatively thin-walled tubes. The wall thicknesses of fittings, flanges, etc. are typically much heavier than those of flexible hoses and expansion joints, so what might be considered an acceptable corrosion rate for a fitting could be unsafe for a hose/EJ. Instead, other more reliable resources should be consulted.

Ask the experts

Rather than consulting a chart or graph that shows general recommendations, a more definitive source of reliable, detailed chemical resistance information is frequently required. These various databases are provided by the organizations and their subsidiaries that do the actual corrosion testing on metal alloys when exposed to various chemicals under various operating conditions. A few of the more popular resources include publications by NACE International, ASM, and Compass publications. Some resources are incorporated into certain codes or standards, such as ASME’s B31.3 Process Piping Guide.

Because these chemical resistance resources provide test results for various chemicals and different operating conditions, it is important that you provide the proper name of the chemical as well as the temperature and concentration at which it is being transferred. The media temperature is needed because chemicals can become more aggressive at higher temperatures. Additionally, the concentration of a chemical can have a dramatic effect on its corrosion rate, and this is where problems can arise. The corrosion rate of some chemicals increases at higher concentrations, but this is not a universal truth; some chemicals are less corrosive at 100% concentration than they are at, say, 8% concentration. That is because water may react with the chemical to create acids, etc. that could readily attack common stainless steels alloys.

Other Considerations

Although these detailed resources provide a depth of information, they usually don’t include name-brand chemicals. For example, if a customer is conveying a heat transfer fluid with the brand name BenzoTherm-30, chances are this product will not be listed in any chemical resistance charts. If this is the case, get the Safety Data Sheet on the product from your customer, as this can often be used to obtain corrosion-resistance information directly from the chemical manufacturer.

It is also important to identify if there are any impurities present in the chemical being conveyed, as these impurities may affect the corrosion rate of the chemical. An example of this would be sour gas. Natural gas can be defined as “sweet” or “sour” depending on the amount of sulfur present. While low-sulfur (sweet) natural gas is rather non-corrosive, the sulfur present in sour gas can aggressively attack stainless steel, so special alloys may be required.

To sum up, remember that we have access to various resources that provide useful corrosion resistance information. If there is any question about whether or not a certain alloy is able to resist a chemical, contact us and let us help. Not only can we consult these resources for information that you can trust, but our in-house Product Analysis Lab can verify if a product showed any signs of corrosive attack. This one-two punch guarantees that our products will perform reliably and safely.

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