Proteksi Inggris
Proteksi Inggris
Proteksi Inggris
NACE RP0178-91
provides good examples of weld defects and their correction.
Another, usually less severe and less commonly occurring, form of concentration cell
corrosion occurs with different concentrations of metal ion. A buildup of metal ions in the
crevice will make the crevice cathodic to the metal outside the crevice (M-M’’+2c). Thus, metal
ion buildup in crevice causes corrosion to occur on the metal outside the crevice. As with
galvanic corrosion, the rates of both of these types of concentration cell corrosion are greatly
affected by the relative sizes of anode and cathode areas.
Dealloying
Dealloying is the selective corrosion (loss) of the metallic constituent from an alloy, such as
in a plumbing fixture. The leached metal may be aluminum, nickel, molybdenum, or zinc. One
example of dealloying is the dezincification of brass. Another example in graphitization, which
consist of corrosion of cast iron in which the metallic constituents are converted to corrosion
products, living in graphite intact.
Erosion - Corrosion
Erosion – corrosion occurs when a abrasive material (e.g. slurry through a pipeline) impinges
on an existing corrosion cell to keep the metal bright and the corrosion active. The scouring of
the surface to remove the accumulation of corrosion products result from wind or water-borne
sand.
Some metals (e.g. gold, silver, and titanium) are essentially unaffected by corrosion in certain
environments. This may be due to the stability of some metal in their metallic states or to
passivity imparted by the formation of protective oxide films. However, these metals and alloys
may not be stable in other environment, so the environment must be taken into account when
choosing corrosion-resistant metal product.
Exfoliation
Exfoliation is an advanced form of inter granular corrosion where the metal delaminates
along grain boundaries. Rolled metal products such as certain types of aluminum alloy plate are
particularly susceptible to exfoliation.
Incompatible environments can accelerate the corrosion process. Thus, aluminum should
not come into direct contact with concrete, because the alkalinity of the concrete will attack the
aluminum.
Water traps are design features that allow rainwater of dew to collect. Since water greatly
accelerates corrosion, structures should be designed so that they do not collect water. Angels and
other shapes that can collect water should be oriented downward. Weep holes be placed where
water collection cannot be otherwise avoided. Condensate water from air conditioners should not
be allowed to run of drip on surface, nor should steam of other vapors be allowed to impinge on
metal surface.
Limited access to work can prevent proper application of coating. All areas to be coated
should be readily accessible for both cleaning and painting. Difficult-to-reach areas are not only
difficult to coat, but may also constitute a safety hazard because of reaching from a leader or
other platform further than can be done more safety.
Titanium
Titanium provides a high strength-to-weight ratio and good resistance to many severe
environments, e.g. seawater, hypochlorite’s and nitric acid. Its surfaces are readily passivated
with an oxide film.
Aluminum and its alloys are light in weight and corrosion resistant in many environments.
They are attacked by strong acids and alkalis.
Stainless Steels
Stainless steels alloys are produced almost exclusively for their corrosion resistance. These
steels should contain at least 11% chromium for effective corrosion resistance.
Weathering Steels
Under mild atmospheric conditions, weathering (low-alloy) steels form protective oxide
films that may defer or eliminate the need for coating. However, these oxide layers may not be
protective in some, particularly marine, environments. In addition, the natural rusted appearance
of weathering steels may not be acceptable aesthetically, and soluble iron compounds that wash
from the surface may produce an objectionable stain.
Galvanizing
Hot-dipped galvanized steel is formed by dipping cleaned (usually by acid immersion) steel
into a molten bath of zinc coating subsequently formed is resistant to corrosion under many
conditions and can be top coated with an organic coating to provide further protection or a
different appearance. Its mechanisms of protection are both galvanic and barrier. This is
discussed in more detail in unit 6. Hot dipping is usually preferred to other methods of
galvanizing, because it produces a thicker layer of zinc than does electro galvanizing (electro
deposition). Also, it is metallurgic ally bonded to the steel. One of the limitations of hot-dipping
is that the component being coated must be small enough to fit into the bath. Nut, bolts, and
other fasteners are sometimes galvanized by tumbling with powdered zinc. Metalizing and zinc-
rich coatings, to be described in Units 2 and 4, are other ways to coat a steel surface with zinc.
Plastics
A plastic is a solid material that is essentially an organic polymer of large molecular weight
which contains hardeners, fillers, reinforcements and other components. At some time during its
manufactures, it is shaped by flow, it is either thermoplastic, i.e. can be repeatedly softened by
head and hardened by cooling, or it is thermosetting, i.e. when cured chemically or by heading, it
becomes an infusible, insoluble material.
Elastomers
Natural and synthetic elastomeric materials (those that substantially return to their original
shape after removing the force causing distortion) lining materials have been successfully used
as liners for primary containment. These include natural and synthetic rubber and polyurethane
coating and sheets. Sheets must be glued or mechanically fastened to walls.
Composites
Composites are combinations of two or more materials (e.g. binder with reinforcing materials
and fillers) differing grossly in form or composition. The different remain as separate entities and
do not merge together, but they do act in concert with each other. One of the most common types
of composites is fiber-reinforced plastics (FRP). Reinforcement may be from cloth, mat of
strands of glass, carbon, or other materials. Fiberglass-reinforced plastics (FGRP) are by far the
most commonly used FRP products. FGRP composites can be used to fabricate process vessel,
piping, floor topping, tank, etc.
Ceramics
Ceramics are products formed by the firing of natural earth materials at high temperatures.
They exhibit high chemical, temperatures, and electrical resistance and are used where these
properties are desired.
Also, if dehumidification result in a 15o F (8oC) dew point depression and humidity no higher
than 35%, blast cleaned surfaces can be left uncoated for significant periods before painting.
The sacrificial anode (galvanic) system of cathodic protection requires no external power
supply, but incorporates anodes of special alloys that generate the necessary direct current by
preferentially corroding by virtue of their natural voltage difference from the protected structure
(Figure 1-54 in appendix).
Because sacrificial anodes are anodes are consumed in generating current, they have a
limited service life. The active anode metals used in cathodic protection are usually magnesium,
zinc, or aluminum of high purity or other special composition.
The impressed current system of cathodic protection utilizes direct current from an external
power source. The positive terminal of the power source is connected to the anodes, and the
negative terminal is connected to the structure to be protected.
The stable anodes used to discharge current have long service. High-silicon cast iron,
graphite, and aluminum are among the most commonly used anode materials. Crap iron, special
lead alloys, platinum, platinum-palladium alloy, Platonized titanium, and Platonized tantalum
alloys are also used. Normally, rectifiers convert available AC power to DC power for the
system. Batteries and solar power can also provide energy for cathodic protection systems where
is inaccessible.
Each of these two types of cathodic protection has its own advantages and limitations. The
differences in the systems are summarized in table 2:
Coatings have many advantages over the other previously discussed methods of corrosion
control. These include:
Ease of application
Ease of storage and handling
Range of acceptable ambient conditions
Economics
Ease of repair
Selection of color, gloss, and texture
As with other methods of corrosion control, they also have limitations that must be
addressed. These include:
Corrosion of metals is a natural process by which the metals are transformed to a more stable
state. There are many forms of corrosion, as well as many methods of corrosion control. A
successful total corrosion program utilizes as many of the available methods as are appropriate
and practical.