Corrosion Control and Welding in Offshore Environments: Strategies for Longevity
Corrosion is one of the most damaging factors in offshore structures that can lead to significant damage and even catastrophic structural failure. Corrosion control is vital for the long-term success of offshore oil rigs and other structures like offshore wind structures.
Welds produced in offshore environments are usually more susceptible to corrosion than the base metal. So, weld corrosion control requires additional consideration because more variables are involved.
What Is Corrosion And Why It’s So Prevalent In Offshore Environments
Corrosion is a natural, chemical process during which metals erode. The refined metals are converted into more stable compounds, like metal oxides, via the action of environmental moisture and oxygen. This is a spontaneous process that is very challenging to prevent.
Water works like an electrolyte to facilitate the ion path, while the metal transfers its electrons from the anodic area to the cathodic area. So, you can think of the corrosion process like a battery, but instead of energy, the end product is corrosion and material erosion. In the presence of moisture, water, and sea chlorides, metals erode by transferring electrons from an anodic area (+) to a cathodic area (-). As a result, the anodic area loses material mass as it dissolves.
The seawater contains salts, like sodium chloride, which drastically exacerbate the corrosion process, which is why offshore structures are at a significant risk of corrosion-related structural failure.
Why The Corrosion Fatigue Is So Critical
Material fatigue is a well-understood concept — structural elements exposed to cyclical stresses and temperature swings can develop cracks. As the cyclical loading continues, the cracks eventually grow enough to cause a fracture.
Corrosion fatigue is far worse. It’s much more unpredictable, resulting in unexpected consequences. Material fatigue coupled with corrosion usually means that the material’s cyclical loading strength is limited by the corrosion. As a result, the material’s ability to withstand cyclical loading is much lower in corrosive environments. In other words, corrosive environments produce cracks faster, which also propagate more quickly, leading to premature failure when exposed to cyclical loading.
Offshore structures, like oil rigs, are exposed to constant cyclical loads from tidal forces, waves, and seismic activity. In addition, high-pressure and high-temperature pipelines are exposed to cyclical thermal loading. Once any pipeline or structural elements develop corrosion, they become more likely to crack upon further loading.
Types of Corrosion on Structures in Offshore Environments
Some of the most common corrosion types occurring in offshore environments are:
- Galvanic corrosion – Also known as bimetallic corrosion, occurs when two dissimilar metals are connected and immersed in a conductive solution (electrolytes, like salt, water, and acids). This corrosion type affects one metal that corrodes while the other one is protected. It’s the most common type of corrosion in offshore environments. The best example would be a stainless steel flange and a carbon steel nut. Since the carbon steel nut is more anodic, it corrodes, while the stainless steel flange is protected.
- Crevice corrosion – Occurs when small volumes of stagnant chloride-rich solution is present at joints between metal surfaces. For example, crevices under washers, fastener heads, surface deposits, threads, lap joints, threaded pipes, and similar areas where the two metals meet.
- Microbial-induced corrosion – Seawater is home to many aerobic and anaerobic bacteria. Both contribute to corrosion as a result of their biological activity as they colonize metal surfaces and form biofilms. These bacteria produce by-products like hydrogen sulfide and organic acids that can contribute to corrosion.
- Intergranular corrosion in stainless steel – Welding stainless steel pipes and other structures can cause stainless steel sensitization due to overheating. The carbon migrates to the grain boundaries and combines with the chromium to form chromium carbides. As a result, grain boundaries become susceptible to corrosion.
- Pitting corrosion – This corrosion type is one of the most dangerous corrosive attacks because even the most resistant alloys like stainless steel, chromium, and aluminum can suffer devastating erosion from it. In addition, pitting corrosion starts like a tiny hole and can go unnoticed. It’s difficult to detect and prevent in time and can lead to complete structure failure. Pitting corrosion starts when seawater droplets settle and leave salt residue on the material’s surface. Chlorides break the protective film on the surface, establishing the anodic area in the process. As a result, the cathodic area next to it causes rapid metal loss in the anodic pit.
Preferential Weld Corrosion (PWC)
Now that we’ve discussed the nature of corrosion, we can explain why welds often have a significantly higher risk of corrosion.
Preferential weld corrosion (PWC) is a condition where the weld metal is anodic to the parent material. As a result, the weld metal will develop corrosion at a higher rate than the base metal. Weld metal can become anodic for many reasons. But, the primary reason is the difference in composition between the weld metal and the base metal, which generates a potential difference (“anodic = + “, “cathodic = -“), leading to corrosion.
Even if you take every step to ensure that the chemical composition of the weld metal matches the parent material, the PWC can still occur. Minor slag inclusions, differences in fluxes, and even minuscule chemical composition differences can cause the weld metal to have a slight anodic nature. In addition, microstructural differences between the base material and weld metal can lead to PWC, as noted by The Welding Institute.
Corrosion Control – Strategic Ways To Prevent Corrosion In Offshore Structures
Corrosion control is critical in keeping offshore structures like oil rigs and wind farms operational. Some aspects of corrosion control are essential during the fabrication process, while others are performed on an ongoing basis. Not conducting proper corrosion control during the welding procedure can make it significantly more challenging to combat the resulting corrosion later on.
Application of Coatings
Protective coatings are the most applied corrosion control methods for offshore structures. There are many coating types, and the selection depends on factors like the exposure type, base metal, and application and economic constraints.
Regardless of the applied coating type, weld surface condition plays a major role in coating effectiveness. Erratic weld surfaces with crevices and uneven weld profiles can make coating application difficult and leave room for error. In addition, a poor weld surface can contribute to the coating getting chipped off, exposing the weld to a corrosive environment.
It’s beneficial to use our automated orbital welder rentals for joining pipes whenever possible. It produces exceptionally clean welds that require no post-welding cleaning, allowing you to cut production time before coatings can be applied. Likewise, using pulsed MIG for welding structural steel in offshore structures, or flux-cored when outdoor conditions require it, can produce smoother welds than stick welding.
Remember how we explained earlier that the preferential weld corrosion causes the weld to corrode instead of the base metal because it’s more anodic? That’s the same mechanism corrosion experts use to their advantage by providing sacrificial metal to draw corrosion instead.
Cathodic protection involves taking a sacrificial metal (galvanic anode), like aluminum and zinc, and physically bonding it with the protected steel structure or using a wire connection. Since aluminum and zinc are significantly more anodic than steel, they corrode instead and protect the steel in the process. Designing cathodic protection systems is a complex task, but they are essential in keeping offshore structures and subsea pipelines corrosion-free.
Weld Overlay Cladding For Interior Pipe Protection
Critical pipelines, like offshore gas risers, that transport highly corrosive agents must be protected from the inside. Instead of using corrosion-resistant alloys (CRA), like stainless steel and specialized nickel alloys, many projects rely on carbon steel pipes cladded with CRA’s from the inside to cut costs.
Cladding lays a protective layer of CRA on the inside of the pipe, giving it added resistance to corrosion at a fraction of the cost compared to using a pipe made from CRA. Our automated TIG cladding systems make a uniform weld layer inside the pipe, producing a clean and efficient corrosion-resistant layer for critical offshore pipelines.
Using Consistent Heat Input And Heat Treatment
In order to produce uniform welds and heat affect zone (HAZ), it’s vital to provide uniform pre-heating, interpass temperatures, and post welding heat treatment (PWHT), according to the welding specification sheet (WPS). This is especially true for high-strength steels, which have a higher risk of weld and HAZ hardness after welding.
Since welds are prone to PWC, their microstructure should be as similar as possible to the base material. So, it’s important to apply a uniform heat input. Otherwise, the weld zone and HAZ microstructure may not be uniform enough, which may lead to unpredictable outcomes, including parts of the weld having a higher anodic potential and resulting in PWC. In addition, some steels can benefit from the correct PWHT application to reduce the chance of PWC.
Qualified engineers design welding procedures with heat input and PWC in mind. However, using less reliable pre-heating equipment like open-flame gear can produce uneven heat input, which may negatively impact the end quality. That’s why it’s vital to use high-quality induction heating equipment to achieve accurate and consistent thermal gradients exactly according to the engineer’s specifications.
The Miller ProHeat 35™ is an advanced induction heating system that allows you to reach pre-heat and PWHT temperatures precisely as specified. You can use it on pipes and other steel structures, and it can be air or water-cooled. The Miller ProHeat 35 is an easy-to-use system that’s significantly safer than open-flame solutions. It doesn’t require any fuel tanks, which solves the issue of their transport, storage, and safety compliances, making it one of the most efficient and straightforward heating solutions for almost every application.
Rent or Lease Your Welding Equipment From Red-D-Arc
Offshore environments are harsh and pose many other challenges besides corrosion. Welding contractors rely on heavy-duty and rugged welding power sources and diesel generators to install, repair, and maintain offshore rigs. Likewise, welding shops can use advanced weld automation equipment to weld parts of offshore equipment that are delivered on-site.
Contact us today, and our experts will help you choose suitable welding equipment for offshore environments. Welding equipment rentals can save you significant resources. Instead of buying costly gear that takes months or years to pay off, renting lets you immediately experience the ROI.
Red-D-Arc Welderentals™ an Airgas company rents and leases welders, welding positioners, welding-related equipment, and electric power generators – anywhere in the world. Our rental welders, positioners and specialty products have been engineered and built to provide Extreme-Duty™ performance and reliability in even the harshest environments, and are available through over 70 Red-D-Arc Service Centers, strategically located throughout the United States, Canada, the United Kingdom, France, and the Netherlands, as well as through strategic alliances in the Middle East, Spain, Italy, Croatia, and the Caribbean. From our rental fleet of over 60,000 welders, 3,700 weld positioners, and 3,700 electric-power generators, we can supply you with the equipment you need – where you need it – when you need it.