Chemical Technology March 2015

Chemical Technology • March 2015

Tough, safe, unrivaled GEMÜ 490 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 Northriding, Randburg Phone: (0)11 462 77 95 info@gemue.co.za www. gemu-group .com GEMÜ 491

Corrosion effects on valves? – Call in the

specialist

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.

For more information

contact Claudio Darpin at GEMÜ Valves Africa

Claudio.darpin@gemue.co.za

Effect of worm hole cor-

rosion on a super duplex

valve disc

Uniform attack corrosion over

the entire surface of a cupro-

aluminium valve disc