Chemical Technology March 2015

Corrosion effects on valves? – Call in the specialist

Tough, safe, unrivaled GEMÜ 490

Uniform attack corrosion over the entire surface of a cupro- aluminium valve disc

Effect of worm hole cor- rosion on a super duplex valve disc

Uniform attack corrosion, also known as general attack corrosion, is the most common type of corrosion and is caused by a chemical or electrochemical reaction that results in the deterioration of the entire exposed surface of a metal. Ultimately, the metal deteriorates to the point of failure. This kind of corrosion accounts for the great- est amount of metal destruction by corrosion, but is considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable. Unlike uniform attack corrosion, localised corrosion specifically targets one specific area of the metal structure, ending up as pitting, crevice corrosion or stress corrosion cracking. This form of corrosion is more dangerous and destructive due to its latent incubation and quick propagation. Pitting is encounteredmost frequently inmetallic materials of technological significance such as carbon steel, low alloy and stainless steels, nickel base alloys, aluminum, copper, and many other metals and alloys. Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area be- comes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure. Corrosion pits will continue to grow, since the interior of a pit is naturally de- prived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an autocatalytic process. A special form of pitting corrosion is worm hole corrosion, which does not spread laterally across an exposed surface, but penetrates at 10 to 100 times the rate of general corrosion, usually at an angle of 90° to the surface. Worm hole corrosion of discs made of Super Duplex can take place, mostly due to poor Super Duplex casting methodology, impurities on the metal surface (e.g., iron particles released when flame-cutting or welding in the proximity), but often due to insuf- ficient passivation. It is essential that valve discs and components made out of Super Duplex are acid-pickled to remove impurities that may lead to such corrosion in service. It is important to have exact knowledge of the working condi- tions involved, in order to design and recommend the most suitable valve technology. GEMÜ not only offers valves and solutions of high quality, but also offers its customers its expertise in designing the best cost/performance solutions for the problem.

GEMÜ 491

Teflon-seated butterfly valve: • Disc made from high alloy steel, titanium or PFA encapsulated stainless steel • Reduced torque values • Dynamic shaped liner in PTFE or TFM • Designed to stand highest corrosion • Meeting highest international standards, such as clean air act (TA-Luft)

GEMÜ VALVES AFRICA (PTY) LTD Stand 379, Northlands Business Park Hoogland Ext. 45

For more information contact Claudio Darpin at GEMÜ Valves Africa at Claudio.darpin@gemue.co.za z

Northriding, Randburg Phone: (0)11 462 77 95 info@gemue.co.za www. gemu-group .com

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Chemical Technology • March 2015