Nobility and galvanic corrosion

The relative nobility of a material can be predicted by measuring its corrosion potential.

The well known galvanic series lists the relative nobility of certain materials in a given environment (e.g. seawater).

A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small anodic area leading to a very high corrosion rate.

Galvanic series in seawater Platinum Gold Graphite Titanium Silver Chlorimet 3 Hastelloy C 18-8 Mo stainless steel (passive) 18-8 stainless steel (passive) Chromium steel >11 % Cr (passive) Inconel (passive) Nickel (passive)

Galvanic series in seawater(continued) Silver solder Monel Bronzes Copper Brasses Chlorimet 2 Hastelloy B Inconel (active) Nickel (active) Tin Lead

Galvanic series in seawater(continued) Lead-tin solders 18-8 Mo stainless steel (active) 18-8 stainless steel (active) Ni-resist Chromium steel >11 % Cr (active) Cast iron Steel or iron 2024 aluminum Cadmium Commercially pure aluminium Zinc Magnesium and its alloys

Erosion corrosionErosion corrosion is an acceleration in the rate of

corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface.

Example of erosion corrosionpump rotor destroyed by erosion corrosion

Aggravating factors: high flow velocity flow disturbances presence of solids multiphase flow …

Example of erosion corrosion

Inside of the stainless steel pump column spools pumping seawater serving in the Arabian gulf.

CavitationCavitation occurs when a fluid's operational pressure drops below the vapor pressure causing bubbles to form and then increases causing them to violently collapse.

Cavitation usually happens… At the suction side of a pump, especially if

operating near the net positive suction head

At the discharge of a valve or regulator, especially when operating in a nearclosed position

At flow expansions •in other processes incurring sudden pressure drops and increases

Fretting Fretting corrosion is damage at contact surfaces

rubbing against each other (under load and in repeated relative surface motion, as induced for example by vibration).

Intergranular corrosionIntergranular corrosion is localized attack along the

grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected.

Causes of intergranular corrosion

• This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries.

Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

Example of intergranular corrosion

Sensitization of stainless steels: Chromium-rich grain boundary precipitates

lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes.

Reheating a welded component during multi-pass welding is a common cause of this problem.

ExfoliationExfoliation is a form of intergranular corrosion associated with high strength aluminum alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains.

failed aircraft component made of 7075-T6 aluminum (picture width = 400 µm)