25 May 2005
CORROSION TESTING WITH COUPON-TYPE TEST SPECIMENS
PURPOSE. Corrosion tests with metal coupon test specimens are used to
determine how corrosive water is to a specific metal. Coupon test specimens are
particularly useful in monitoring the effectiveness of chemical corrosion control
programs. The extent of both general and localized corrosion can be determined. Also,
examination of the specimens as they are removed from the system can provide
information regarding deposit accumulation or microbiological slime growth. Corrosion
test coupons are usually used as corrosion test specimens for open and closed cooling
systems, closed hot water systems, and domestic water systems. See ASTM Standard
Test Method: Corrosivity of Water in the Absence of Heat Transfer (Weight Loss
Description of Coupons. The coupons are metal tags of various sizes.
Generally, low-carbon steel specimens are used as the most susceptible metal to
corrosion; however, copper, stainless steel, brass, and other metals are available for
special studies. The specimens are prepared and cleaned to ensure uniformity and
weighed. The general corrosion rate is determined after exposure. Coupons are
cleaned and re-weighed. Corrosion rates are calculated based on the weight loss, time
of exposure, and the area and specific gravity of the coupon, and are expressed in mils
per year (mpy).
Test Conditions. Since the goal of the corrosion testing program is to
provide information regarding conditions within the system, the corrosion coupons
should be exposed to conditions that reflect those in the system as a whole. Water
velocity and temperature will significantly affect the corrosion rate. Corrosion rates can
be significantly increased when velocity is either too low or too high. For best results,
water velocity flowing by the test specimen should be 0.90 to 1.5 meters per second (3
to 5 feet per second), and both adjustable and measurable. Temperature also affects
the rate of corrosion, with higher temperatures usually increasing corrosion; therefore, in
cooling water systems both the cold supply water and the hot return water should be
evaluated with corrosion coupons. When only one test rack can be installed, the warm
water return at the tower should be used to provide an average hot water temperature.
Test Length. Expose corrosion coupons for a minimum of 30 calendar
days and a maximum of 90 calendar days, except for special tests. Although spot
checks are useful, a regular schedule will determine trends and recognize changes
within the system. When corrosion test specimens of different metals are installed in the
same corrosion test rack, the more noble or resistant metal should be downstream of
less resistant metals.
Receipt of Specimens. Specimens are shipped in vapor-inhibited bags.
Do not remove them from the bags until the specimen is to be installed. The vapor-