FORMS OF CORROSION

Dr. Munther Issa Kandah

Chemical Engineering Department Jordan University of Science & Technology

UNIFORM ATTACK

It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface.

The metal becomes thinner and eventually fails.

– A piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface.

– Anodic and cathodic areas keep shifting , so corrosion is spread more or less evenly over the entire surface of the metal.

This 40 year old sample of 8 in. schedule 80 pipe, while clearly containing deposits of iron oxide, shows very even wall loss and long remaining service life.
Combating Uniform Attack

The choice of more suitable materials, inhibitors , protective coatings , or combinations of these expedients.



GALVANIC OR TWO-METAL CORROSION

When two dissimilar metals are in contact with each other (or otherwise electrically connected) and exposed to a conductive solution, a potential is set up between these two metals and a current flows.

Galvanic Corrosion :

An extremely common problem area due to the failure to install galvanic insulators between carbon steel pipe and either brass valves or copper pipe.

Standard emf series of metals

Corrosion on the less- corrosion-resistant metal is usually increased and attack on the more resistant material is decreased, compared to the behavior of these metals when they are not in contact.

Example:

Galvanized steel hot water tanks failed where copper tubing , from an external gas heater , was joined to the tank.

Also, long tubes of aluminum supported by steel.

Area effect

An unfavorable or bad area ratio consists of a large cathode and a small anode, because the current density (A/cm2) is greater for a small electrode than for a larger one.

The greater the current density of the current leaving an anodic area, the greater is the corrosion rate.

Under certain conditions, as shown here, it may also exist at steel to steel contact points – where new pipe meets old, and where different ground potential exists

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