One of the most popular stainless steel alloys used in process equipment fabrication is 316L. It's able to withstand high heats, resists corrosion and rouging fairly well, and is durable and long-lasting. While generally suitable for many applications, this alloy and others like it aren't perfect. In some applications, it may give way to corrosion due to environmental conditions, reactions with the substances introduced to the equipment and other factors.
To address these shortcomings, there was a push in the 1970s and 1980s to develop new alloys with higher levels of chromium and molybdenum, which have been found to be particularly helpful in protecting against corrosion in offshore drilling operations and other circumstances where seawater is a major concern.
One of these new alloys and one of the first super duplex stainless steel was Zeron 100, according to Stainless Steel World. Originally created specifically to be used in seawater applications, it has a high molybdenum content (3.5 percent, as compared to 316L's 2.2 percent by weight) and incorporates nitrogen. Zeron 100 also contains a good amount of nickel, which is known for its corrosion resistance, though also for its costliness.
"One of the first super duplex stainless steel developed was Zeron 100."
Zeron 100's strength allows for thinner walls
Zeron 100 contains 8 percent nickel by weight in its cast iron form and 7 percent in its wrought iron form, but its strength and temperature resistance allow for thinner walls than would be needed when using most other stainless steel alloys, which offsets the higher price.
The super duplex alloy's strength was the reason engineers working to install a membrane filtration system at the Syd Arne reservoir, an offshore oil field off the coast of Denmark, chose Zeron 100 for its pipework, according to Offshore Magazine.
The engineers wanted to control the weight of the system, and their ability to incorporate thinner pipes, smaller diameters and reduced valve sizes made this possible. Since Zeron 100 can handle high pressures and velocities, it was ideal for the system, which required high pressure pumps and a constant flow of seawater through the system.
Nitrogen contributes added benefits
The ability to control nitrogen levels in alloys is a relatively recent development in metallurgy, according to the British Stainless Steel Association. This newfound technique was a major contributor to the development of new alloys and helped give way to the introduction of duplex and super duplex stainless steels.
The nitrogen incorporated into Zeron 100 has several benefits. Nitrogen helps prevent pitting and corrosion, increases strength and makes the metal more weldable – a recurring concern with other stainless steels, which tend to lose corrosion resistance after welding, according to the Water Research Foundation.
Zeron 100 has a pitting resistance equivalent number of greater than 40, which many engineers likely would be happy to see, considering other popular alloys like 316L and 904L have PRENs of 24 and 35, respectively. While it's not the highest PREN around – Hastelloy C-276, for example, comes in at 67 – fabricators often find that Zeron 100's ability to fight against pitting combined with its cost-effectiveness offers the best of both worlds.
Zeron 100's ability to resist corrosion in saline or brine environments is what brought the alloy to the attention of the City of Hollywood, Florida. Unfortunately, the city discovered these benefits after it had already installed a well system primarily composed of 316L stainless steel.
Not long after the system was implemented, it began to rust and pit, according to Water Quality Products. Within a year, the pipes had more than 20 leaks, primarily caused by anodic action and microbiological organisms. The pipes' J-bends sustained the bulk of the damage, where water was allowed to collect and wear down on the metal over time. Sitting water can commonly lead to crevice corrosion, which is exactly what happened in Hollywood, Florida.
It didn't take long for the city to reevaluate its decisions to get to the bottom of the problem. City officials determined that the biggest mistake they made was not considering other types of stainless steel; 316L can't withstand the high chloride levels present in the water treatment system, which is necessary to prevent biofouling.
Zeron 100 has been shown to withstand high chloride levels. Stainless Steel World noted that it doesn't show any signs of chloride stress corrosion cracking when tested with 3 percent sodium chloride solution at 250 degrees Celsius. Stainless steels in the 300 series commonly cannot compete with this. Hastelloy C-276 is another contender, though it comes at a higher price point.
The City of Hollywood identified that a super duplex stainless steel would the answer to their problems, and began testing Zeron 100 to replace the highly corroded J-bends. It determined that an all-out replacement of the system wouldn't be prudent right away, but intends to incorporate more Zeron 100 as parts wear out. It noted in Water Quality Products that, in hindsight, starting out with Zeron 100 could have made the project much more successful.
When it's time to upgrade equipment or bring in additional functions to your operation, it's crucial to consider the materials used for fabrication. Choose the wrong one, and you could wind up with ineffective equipment and escalating maintenance costs.
Every situation, application and environment is different, so there's no one-size-fits-all metal alloy that suits everyone's needs. As such, it's important to confer with an experienced engineer who knows the pros and cons of various metals and can tell you which options would be best for you. If you're considering bringing a new shell and tube heat exchanger into your operation, reach out to the helpful engineers at Enerquip to determine which materials best suit your needs.