Page 91 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: LLL Revised Pages
Encyclopedia of Physical Science and Technology EN002E-49 May 17, 2001 20:13

Batch Processing 55

Embrittlement is a reduction in the strength of met- replaced at intervals. To obtain complete protection, the
als caused by hydrogen and caustic substances, probably current density must make the anode potential equal to its
due to reactions that decarburize steel, thus disintegrat- open circuit potential, at which point no net corrosion can
ing the grain boundaries and promoting the collapse of occur. Polarization curves can be used to estimate current
the crystalline structure. The embrittling effect of hydro- density requirements for cathodic protection.
gen in steel is reduced by the use of molybdenum and
chromium to the extent illustrated in the Nelson diagram.
2. Galvanic Protection
Corrosion is a reaction, chemical or electrochemical, of
the material with the environment. Corrosion resistance This method uses a more active metal than that in the struc-
often determines the selection of materials for a process. ture to be protected, to supply the current needed to stop
Alloys used in industrial service may require protection corrosion. Metals commonly used to protect iron as sac-
and are selected on the basis of the environment and design rificial anodes are magnesium, zinc, aluminum, and their
requirements for each piece of equipment. Corrosion and alloys. No current has to be impressed to the system, since
past service data available from materials of construction this acts as a galvanic pair that generates a current. The
and equipment manufacturers are valuable to ensure sat- protected metal becomes the cathode, and hence it is free
isfactory results and long life of plant equipment. Design of corrosion. Two dissimilar metals in the same environ-
must have the goal of preventing corrosion in some envi- ment can lead to accelerated corrosion of the more active
ronments. It should consider the materials and their treat- metal and protection of the less active one. Galvanic pro-
ments (liners, coatings, and other alternatives) to minimize tection is often used in preference to impressed-current
trappedmoisture,introductionofanewcorrosivemedium, technique when the current requirements are low and the
crevices, and any factors promoting corrosion. Corrosion- electrolyte has relatively low resistivity. It offers an advan-
resistant materials or methods of protection must be se- tage when there is no source of electrical power and when
lected for each exposure condition and within prescribed a completely underground system is desired. Probably, it
economic limits. Laboratory testing can serve as a guide is the most economical method for short life protection.
in this selection, but exposure under actual conditions is
necessary in many cases.
3. Anodic Protection
Most corrosion is electrochemical, originating with the
formation of galvanic cells and the accompanying flow Active metals such as aluminum, titanium, and high-
of electrical current. In a metallic medium two dissimi- chromium steels become corrosion resistant under oxidiz-
lar electrodes may exist because of differences in energy ing conditions because of a very adherent and impervious
levels, probably due to disordered or stressed areas in the surface oxide film that, although one molecule thick, de-
microstructure; differences in composition; or differences velops on the surface of the metal. This film is stable in
in concentration in the electrolytic environment. The elec- a neutral medium, but it dissolves in an acid or alkaline
trode with the higher energy potential becomes the anode environment. In a few cases, such as certain acid concen-
and suffers corrosion; the cathode is protected. Galvanic trations, metals can be kept passive by applying a carefully
corrosion may occur in three different cell types: stress controlled potential that favors the formation of the pas-
cells, composition cells, and concentration cells. In each, sive surface film. The ability to keep the desired potential
corrosion is produced because one half of a galvanic cou- over the entire structure is very critical in anodic control.
ple acts as the anode, and the other half, with a lower If a higher or lower potential is applied, the metal will
electrode potential, as the cathode. Only the anode is cor- corrode at a higher rate, possibly higher than if it is not
roded, when it is in electrical contact with a cathode. protected at all.
Corrosion can be prevented or reduced significantly by
three electrical means.
C. Stress and Fatigue
In batch processing, the reagents and hence the equipment
1. Cathodic Protection
are subjected to cyclic stringent changes in temperature,
The metal is forced to behave as a cathode; thus, it has no pressure,andconcentration,duetothekindofphysicaland
anode areas and does not corrode. This can be achieved chemical changes involved. Thermal and pressure stresses
in two ways. The first is to apply a large dc current to the arise from the temperature and pressure gradients to which
corroding metal, which lowers the metal activity to below the materials are subjected, since the internal and external
that of hydrogen; the second is providing for an electrode layers of metal are subjected to entirely different con-
that acts as an anode. The anode may be inert material, ditions at the same time. Stresses result even when the
such as graphite or scrap iron, which deteriorates and is gradients are very small, if free expansion or contraction