Nace CP 1, Course Manual

Page: 263 / 535

NACE GLOSSARY OF CORROSION-RELATED TERMS
©
2002, NACE International. This publication may not be reprinted without the written consent of NACE International.
Page 17 of 18
T
TACK COAT
A thin wet coat applied to the
surface that is allowed to dry just
until it is tacky before application
of a thicker wet coat. (Use of a
tack coat allows application of
thicker coats without sagging or
runs.)
TAFEL PLOT
A plot of the relationship between
the change in potential (E) and
the logarithm of the current
density (log
i ) of an electrode
when it is polarized in both the
anodic and cathodic directions
from its open-circuit potential.
TAFEL SLOPE
The slope of the straight-line
portion of the E log
i curve on a
Tafel plot. (The straight-line
portion usually occurs at more
than 50 mV from the open-circuit
potential.)
TARNISH
Surface discoloration of a metal
resulting from formation of a film
of corrosion product.
THERMAL SPRAYING
A group of processes by which
finely divided metallic or
nonmetallic materials are
deposited in a molten or
semimolten condition to form a
coating.
THERMOGALVANIC
CORROSION
Corrosion resulting from an
electrochemical cell caused by a
thermal gradient.
THROWING POWER
The relationship between the
current density at a point on a
surface and its distance from the
counterelectrode. The greater
the ratio of the surface resistivity
shown by the electrode reaction
to the volume resistivity of the
electrolyte, the better is the
throwing power of the process.
TOPCOAT
The final coat of a coating
system. [Also referred to as
Finish Coat .]
TOUCH POTENTIAL
The potential difference between
a metallic structure and a point
on the earth’s surface separated
by a distance equal to the normal
maximum horizontal reach of a
human (approximately 1.0 m [3.3
ft]).
TRANSPASSIVE
The noble region of potential
where an electrode exhibits a
higher-than-passive current
density.
TUBERCULATION
The formation of localized
corrosion products scattered over
the surface in the form of knob-
like mounds called tubercles.
U-V-W
ULTIMATE STRENGTH
The maximum stress that a
material can sustain.
ULTRAHIGH-PRESSURE
WATER JETTING
Water jetting performed at
pressures above 170 MPa
(25,000 psig.)
UNDERFILM CORROSION
[See Filiform Corrosion. ]
VEHICLE
The liquid portion of a formulated
coating material.
VOID
(1) A holiday, hole, or skip in a
coating. (2) A hole in a casting
or weld deposit usually resulting
from shrinkage during cooling.
WASH PRIMER
A thin, inhibiting primer, usually
chromate pigmented, with a
polyvinyl butyral binder.
WATER CLEANING
Use of pressurized water
discharged from a nozzle to
remove unwanted matter (e.g.,
dirt, scale, rust, coatings) from a
surface.
WATER JETTING
Use of standard jetting water
discharged from a nozzle at
pressures of 70 MPa (10,000
psig) or greater to prepare a
surface for coating or inspection.
WEIGHT COATING
An external coating applied to a
pipeline to counteract buoyancy.
WHITE METAL BLAST
CLEANED SURFACE
A white metal blast cleaned
surface, when viewed without
magnification, shall be free of all
visible oil, grease, dust, dirt, mill
scale, rust, coating, oxides,
corrosion products, and other
foreign matter. [See NACE No.
1/SSPC-SP 5.]
WELD DECAY
Intergranular corrosion, usually of