How to Choose Water Pipe Stainless Steel?

Most modern buildings’ water supply pipes are hidden so in the event of tube material selection error, the cost of solving the problem can be extremely high. Therefore, the selection of the right material for water supply pipes is very important. They can be made from several materials, stainless steel being one of them. Metallic Steel confirms that stainless steel pipes compare favorably to other materials.

The most commonly used stainless steel pipes in the water industry are 304 and 316 stainless steel. 304 stainless steel water pipe’s tensile strength is 2 times that of ordinary steel pipe. The strength of the material determines whether the water pipe can be strong and durable, safe and reliable.

The working pressure of a high building water supply system is generally higher than 6Mpa, and the pipe pressure is higher. Unlike other materials, stainless steel pipe can reach more than 10Mpa, especially for a high-rise water supply.

The thermal conductivity of stainless steel pipe is the 1/25 of the copper tube, thereby also making it suitable for conveying hot water.

One of stainless steel’s most prominent advantages is its corrosion resistance, which helps ensure durability, and long term performance.

The rising cost of copper and global supply chain headaches have led more municipal water system owners to switch to using stainless steel piping. On the surface, stainless steel is a straightforward choice; it’s less expensive, has similar corrosion-resistant properties, and is lighter than copper. However, this analysis overlooks copper’s unique antimicrobial property. Stainless steel doesn’t have the same contact-killing effect on microorganisms, such as bacteria, algae, fungi, etc.  As a result, stainless steel piping can prematurely fail due to microbiologically influenced corrosion (MIC).

Although 316 austenitic stainless steel has comparable, albeit lower, chloride resistance as the copper alloys used in water systems, the same cannot be said about the leaner, less expensive austenitic grades. This too, has led to early failures.

Naturally, these setbacks come as unwelcome surprises to many operators and installers with plenty of finger-pointing as a result. However, this doesn’t mean that stainless steel is the wrong choice.  It means the differences between alloys must be recognized and considered when building the system.

Two Corrosion Causes, Two Corrosion Types

As mentioned above, the two most typical causes for these early failures are MIC and chlorine. With MIC, commonly present microbes will feed on nutrients present in the water. One particular microbe family, referred to as sulfate-reducing bacteria (SRB), is a significant cause of MIC. While they do not themselves attack the metal, they produce enzymes that accelerate the reduction (conversion) of sulfate compounds in the water into corrosive hydrogen sulfide (H2S). Both copper alloys and the austenitic stainless steel are susceptible to attack with even the smallest amount of H2S present. This is the danger of MIC, an otherwise harmless chemical environment can become damaging to a resistant metal alloy if certain bacteria colonies are allowed to grow on its surface.

Under normal conditions, 316 stainless steel can resist up to concentrations of up to 1000 parts per million (ppm) of chlorine, but the design of the system can create conditions where it only requires around 5ppm to begin the corrosion process. The chlorine attacks the chromium oxide (Cr2O3) that makes up the passive layer and once it penetrates, the bare metal surface is free to corrode.

Whether it’s caused by bacteria or chlorine, localized corrosion takes two primary forms: pitting and crevice corrosion. It occurs when the passive layer is weakened by the acidic environment – with compounds like H2S or Cl–. Once the passive layer is penetrated, it allows contact between the acid and the easily corroding iron in the base metal.  Once the corrosion starts, it continues further into the metal, forming pits. The local environment inside of a pit becomes far more corrosive than outside the pit because the aggressive chemicals inside the pit are typically not flushed out by the flow in normal operation. If not detected, the pits will eventually penetrate fully through the pipe, causing it to leak and require replacement.