Corrosion considerations for flexible metal hoses and expansion joints
There are many industrial applications requiring flexible hoses and expansion joints where the service conditions preclude the use of rubber, plastic, composite, or other non-metallic materials of construction. In these applications, flexible piping components can be made using various metal alloys which provide exceptional service life and value.
^ Photo: Texas A&M Engineering
Article by Frank Caprio, Corporate Trainer, Hose Master
Internal and external corrosion
The first step in alloy selection is to determine the source of any potential corrosion. While corrosive attack may be initiated by the media running through the metal hose, it is also possible that corrosion can initiate from external sources.
If a hose assembly is used in a potentially corrosive environment, then it should be made using an alloy that is resistant to the corrosive agent unless it can somehow be shielded from exposure to that corrosive. This can be tricky, as many covers do not provide adequate corrosion protection, and may even exacerbate the problem. For example, there have been instances where flexible polyvinyl chloride (PVC) covers have been applied onto stainless steel-corrugated dock hoses as a means to protect them from the salt water environment. Over time, these covers can begin to degrade, releasing chloride-containing compounds that can attack the stainless steel hose. External corrosion can also be caused by media that drips or sprays onto the exterior surfaces of the connector.
If the media being transferred through the hose or expansion joint is corrosive, then proper alloy selection is critical. Here, it is important to remember that although the product being conveyed may not be corrosive, it may contain impurities that can cause problems. A good example here would be steam transfer. Boiler water may contain various water treatment chemicals such as anti-scaling or anti-foaming agents, and water-softening chemicals, all of which can be corrosive if allowed to concentrate in the system. Natural gas may also contain sulfur-based impurities that can attack commercial stainless steels. This ‘sour gas’ can lead to critical safety issues if system corrosion results in gas leaks. A detailed analysis of the medium may be required in order to identify any corrosive impurities that may be present.
Once potential corrosive agents have been identified, the next step is to determine which alloys will best withstand any corrosive attack. Most alloy producers provide detailed specification sheets for the alloys they offer that give valuable insight as to the suitability of a given alloy when exposed to certain chemicals. However, in corrosive applications, industry resources that show real-life test results might provide more reliable data. Various databases are published by organizations which perform corrosion testing on alloys, analyzing their resistance to different chemicals under various operating conditions.
Some of these resources are referenced in industry standards and specifications. When using these databases, not only will you need to know the name of the chemical being transferred, but also the temperature and concentration percentage at which it is being conveyed, as these variables can have a dramatic effect on the corrosion rate. For example, sodium hydroxide is generally non-corrosive at low temperatures and concentrations, but becomes aggressively corrosive to stainless steel as the temperature and/ or concentration increases. This is also true for many water-treatment chemicals. Conversely, some chemicals may exhibit reduced corrosion at high concentrations, so caution is key. There are a few important considerations when consulting these corrosion resistance charts. First, they typically do not include any corrosion resistance data for name-brand chemicals or mixtures of multiple chemicals. If name-brand chemicals are being transferred, the chemical manufacturer should be consulted for corrosion resistance data. Secondly, certain corrosion resistance information may be product specific. In other words, corrosion charts that can be found in the back of catalogs for fittings, valves, pipe, etc. should not be used as a reliable corrosion guide for flexible hoses.
While these charts are fine to use as a guide for the products in the catalog, they can be misleading. Although a chart may give an ‘acceptable’ rate of corrosion for those specified products, that same rate may not be acceptable for a flexible metal hose, which is formed using relatively thin-walled corrugated tubing. Incidentally, be wary of corrosion-resistance information found online and make sure that all data is published by a reliable source. Caveat emptor: Buyer beware, especially when the information is free. It is important to remember that, if a metal hose or expansion joint is attacked by a chemical, it is seldom because the alloy is defective. In most cases where corrosion is present, either the incorrect alloy was selected, or the alloy was exposed to unspecified chemicals to which it was not chemically resistant.
“If the media being transferred through the hose or expansion joint is corrosive, then proper alloy selection is critical. Here, it is important to remember that although the product being conveyed may not be corrosive, it may contain impurities that can cause problems”
Spotting the signs
- There are many ways to spot the early stages of corrosion in metal hoses and expansion joints. Some recommendations include:
- Inspect items regularly for signs of any leaking product, chemical residue on the outer surface of assemblies, and any signs of general or pitting corrosion.
- Re-test the assemblies periodically, annually at minimum, to make sure the product can safely handle the required pressure. Hoses should be retagged with the test date and pressure.
- In critical service applications, it is advisable to replace assemblies at regular intervals even if no noticeable signs of corrosion are present. For example, chlorine loading and unloading hoses are recommended to be replaced every two years.
- Look for any signs of braid wear or bulging, hose squirm, dents or other physical damage, as these compromise the assembly’s rated pressure or its ability to withstand fatigue.
- Certain expansion joint designs can incorporate multiple plies, sometimes including detection devices that will indicate if the inner-most ply is no longer retaining pressure.
Various corrosion issues
There may be specific application-dependent factors that can accelerate corrosive attack. Stress corrosion cracking (SCC) can create problems when stainless steel hoses are subjected to high tensile stresses in certain corrosive environments, which are often chloride induced. SCC can be hard to predict and difficult to detect, as the cracking tends to propagate through the metal, leaving little evidence of attack on the surface. Another application-specific form of corrosion is microbiologically-influenced/-induced corrosion (MIC). While MIC does not cause corrosion, it can greatly increase the rate of attack where corrosion cells exist. MIC occurs in certain applications where specific sulfur-reducing bacteria create a slime that adheres to the metal surface and penetrate small imperfections in the oxide layer of the alloy, causing accelerated corrosion. These bacteria can also produce by-products that further accelerate the rate of attack. Applications that are particularly susceptible to MIC include heat exchangers, or hoses that are buried underground or beneath containment berms. It is crucial to pay special attention to materials selection and fabrication techniques in order to avoid these application-specific issues.
“Although a chart may give an ‘acceptable’ rate of corrosion for those specified products, that same rate may not be acceptable for a flexible metal hose, which is formed using relatively thin-walled corrugated tubing.”
Improving corrosion resistance
Stainless steels are able to resist corrosion due to the formation of the chromium oxide layer on the surface of the alloy, which is a protective or ‘passive’ layer that resists further oxidation, a type of corrosion, by chemicals. Sometimes, fabrication techniques such as forming, welding, and grinding can compromise this oxide layer, leaving the metal more vulnerable to oxidation. In certain instances, it may be necessary to reform this oxide layer through passivation. Passivation is accomplished by thoroughly cleaning the material, then submerging it in a dilute acid bath, which dissolves any embedded iron on the alloy’s surface. This is followed by reforming a new chromium oxide layer. There are different passivation methods depending on the alloy and the intended application. Fabrication techniques can also affect the alloy’s ability to resist chemical attack, as substandard welding or finishing practices can compromise the corrosion-resistance of the alloy. This is why flexible hoses and expansion joints should only be fabricated by skilled professionals. Reducing the heat-affected zone (HAZ), minimizing oxidation of welds through the proper use of purge gas, and avoiding any carbon contamination of stainless alloys during fabrication will increase the service life and corrosion-resistance of these products. For flexible metal hoses, it is also important to avoid the formation of any crevices inside the hose where chemicals can collect and cause problems. There are several metal hose fabrication techniques that can be used to provide a smooth, crevice-free, hose-to-fitting transition, thus avoiding any chemical entrapment. Another fabrication method that can help avoid corrosion issues is to reduce the heat input by using a brazing technique to attach fittings rather than welding. The brazing filler metal – typically containing a high silver content – melts at a much lower temperature and does not require that the base metals be heated to their melting point as is required during fusion welding. This greatly increases the strength of the fitting attachment and enhances corrosion resistance. Brazing also permits the joining of dissimilar alloys that cannot be welded, for example, attaching copper or bronze alloy fittings to a stainless steel hose. However, brazed joints have temperature limitations, as the brazing filler metal begins to soften at elevated operating temperatures.
“Applications that are particularly susceptible to MIC include heat exchangers, or hoses that are buried underground or beneath containment berms. It is crucial to pay special attention to materials selection and fabrication techniques in order to avoid these application-specific issues.”
There are many other factors unrelated to corrosion that can drastically reduce service life, such as braid wear or bulging, over-bending, physical damage, and squirm. To avoid and detect these problems, seek assistance from your supplier is the recommended course of action.
“Passivation is accomplished by thoroughly cleaning the material, then submerging it in a dilute acid bath, which dissolves any embedded iron on the alloy’s surface. This is followed by reforming a new chromium oxide layer. There are different passivation methods depending on the alloy and the intended application.”
There is no one single alloy that is universally resistant to all chemicals in all concentrations, so it is advisable to look at a number of different alloys to determine which will best handle the required operating conditions at a competitive price. In corrosive applications, safety is paramount. Make no compromise in safety by using a less expensive alloy in hopes that it will last ‘long enough’. If you are not sure if an alloy will work, ask the manufacturer to assist you with proper alloy selection. In the event of a product failure, it is critical that an analysis of the component is performed by qualified personnel in order to properly identify the mode of failure, so that corrective action can be taken. Too often, incorrect assumptions are made as to the cause of a failure, when the actual cause – and the required remedy – is much different.
About the Author
Frank Caprio, Corporate Trainer and Major Market Specialist at Hose Master, LLC, has more than 35 years of experience in hydraulic, industrial, metal hose, and expansion joint products and applications, and is the ‘Dean’ of Hose Master’s training program, and Hose Master University.
He is national recognized as a leading authority in metal hose, has authored various articles for indus-try publications, and has become a sought-after source for industry facts and trends.