Whether buried, above ground or submersed in seawater, pipelines and their flanges must be protected against corrosion. Although modern alloys like super duplex would seem to be the obvious answer, it is not always possible to use them, especially in field pipelines. They should not be welded above 2800C, for example, or they can become brittle. Yet even if corrosion-resistant alloys have been selected, it isn’t always enough. Maintenance is essential to check corrosion hasn’t occurred, even where duplex is used. Pipelines transport potentially hazardous liquids and gases, and corrosion around the pipe fittings will weaken the entire structural integrity of the line. This often starts at molecular level, during †or shortly after - welding. But how will you know, unless you come back to check? There are four main ways to control corrosion in modern pipelines. These are protective linings and coatings, cathodic protection, inhibitors, and choosing resistant materials. Protective coatings and linings are principal tools for defending pipelines against corrosion. These are often used with cathodic protection, since welding high-strength alloy flanges to carbon steel pipelines can cause galvanic corrosion along the welds. Using protective coatings is the cost-effective way to marry standard steel pipes to high-alloy pipe fittings. Cathodic protection is widely used in pipelines. It’s a method that uses electrical current to counteract bimetallic (i.e. galvanic) corrosion across two unequal metal surfaces - for example, a super duplex flange welded to a carbon steel pipe. It is essentially restricted to areas underwater or underground, where there is an electrolyte. If ordinary stainless alloys are used in a chemically corrosive environment, corrosion inhibitors should be used. These compounds decrease environmental attack on pipeline metals, and are widely used in the field. However, the best way to protect your pipe flanges is to make sure they’re made of the most corrosion-resistant alloys possible. Super duplex of the quality we supply at Chemipetro is still one of the best alloys for pipelines, and any welding problems are easily overcome.
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