Page 35 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

STRAY CURRENT CORROSION 13

The pores act as anodes and the stainless steel bottom as the cathode. The corrosion
current can be multiplied by a factor of 10–20 resulting in a quick penetration of the
anodic sites. This failure shows (i) the cathodic control, which limits the corrosion
current in several aqueous solutions is not operational; (ii) the distance factor and
its effect on the failure is evident as the perforation occurred near the welding junc-
tion in low-conducting solutions. Galvanic corrosion can be prevented or reduced by
(i) avoiding contact between metals with different potentials; (ii) protection by metal-
lic, nonmetallic, nonorganic, organic (paints, lacquer) coatings is recommended; (iii)
large cathodes and small anodic surfaces should be avoided; (iv) electrochemical
testing and determination of polarization characteristics of all the components as
recommended in Pourbaix diagrams of the system should be consulted; (v) use of
corrosion inhibitors should be considered; (vi) prediction of the anodic and cathodic
components of the galvanic cell and inversion of the polarity of the cell should be
considered; (vii) cathodic protection is the only complete protection of the metallic
surface.

1.2.5 Testing of Galvanic Corrosion

Corrosion testing for galvanic corrosion may be predicted by ASTM standards in
the form of potential measurements. The driving force for galvanic corrosion is the
potential difference between the anode and cathode. The galvanic currents between
two dissimilar metals are measured using a zero resistance ammeter (ZRA) for a
chosen length of time. The ratio of anode to cathode areas is 1:1.

1.3 STRAY CURRENT CORROSION

In the past, stray currents resulted from DC-powered trolled systems, which are now
obsolete. An electric welding machine on board the ship with a grounded DC line
located on shore will cause accelerated attack of the ship’s hull as the stray currents
at the welding electrodes pass out of the ship’s hull through the water back to the
shore. Houses in close proximity can dramatically corrode at the water line. The pipe
in one house may be heavily corroded while the pipes in a neighboring house may be
intact.
The majority of stray current problems occur in cathodic protection systems. The
current from an impressed current cathodic protection system will pass through the
metal of a neighboring pipeline at some distance before it returns to the protected
surface. Increased anodic corrosion is frequently localized on the pipe at the zone
where the current leaves the pipe back to the protected steel tank.
Stray current flowing along a pipeline very often will not cause damage inside
the pipe, because of the high conductivity of the electric path compared with the
electrolytic path. The damage occurs when the current reenters the electrolyte and
will be localized on the outside surface of the metal. If the pipe has insulated joints and
the stray current enters the internal fluid, localized corrosion on the internal side of
the pipe will occur. The best solution to avoid this mode of corrosion is the electrical