Civil Engineering Reference
In-Depth Information
loss is evenly distributed over the test surface, making it possible to present the data
as a surface penetration rate. On metals that generally corrode uniformly (i.e. copper,
copper alloys, Pb / Sn Solders, etc.), this is acceptable and can be used to calculate an
approximation of service life. This type of analysis may, however, be inappropriate for
pitting surfaces or other localized forms of corrosion, where a uniform representation
of metal loss is likely to grossly overestimate remaining service life. In such cases,
penetration of the pipe wall will occur in a small fraction of the time predicted by a
uniform assumption of corrosion.
There is no single standard regarding coupon geometry, materials, or exposure
protocols in drinking water systems. While some coupon techniques have been devel-
oped specifically for drinking water distribution systems, others have been borrowed
from different industries. The American Society for Testing and Materials (ASTM)
has certified several methodological variants for use in the evaluation of metal loss on
coupon exposures. The most widely used technique (ASTM D2688-83 method B)
relies on flat rectangular coupon specimens mounted on nonmetallic stems and inserted
directly into the flowstream of the pipe, usually at an elbow or tee. This technique can
be used to make relative assessments of corrosion at different locations in a distribution
system, and for comparative analyses of corrosion inhibitors. However, since the hy-
draulic flow lines around a flat coupon positioned midpipe are substantially different
from the flow lines at the pipe wall, this coupon technique may be inappropriate when
a precise estimate of piping corrosion rates is required. Also, it is frequently difficult
to obtain flat coupons that are truly representative of pipe materials. Table 21-4 pre-
sents a comparative summary of published coupon protocols, including geometry and
exposure conditions. Figure 21-10 provides schematics of some of the coupon expo-
sure rigs and mounting hardware.
A rigorous coupon evaluation involves measuring weight loss over an extended
period of time and requires coupon sacrifices at multiple points in the exposure cycle.
A weight loss against time curve is drawn, and the corrosion rate at any point is the
gradient of that curve at that exposure duration. Experience has shown that on most
metal surfaces corrosion rates change over the course of the exposure, with the highest
corrosion rates occurring at the beginning of the exposure and then rapidly decreasing
to a lesser and more constant rate. Hence, any comparison of corrosion rates for a
particular metal must be standardized to a specific exposure duration. Figure 21-11
presents a weight-loss curve typical of cast-iron coupon exposures.
21
In this example,
the weight-loss gradient at 120 days was interpreted as the stable long-term corrosion
rate. Note that the 120-day gradient is approximately one half the gradient at 30 days.
Electrochemical Techniques
Corrosion is an electrochemical (EC) process, and
electrochemistry can be a powerful tool in its assessment. EC techniques can determine
the underlying rate of corrosion as well as characterize the surface reactions that con-
trol or limit it. In the past decade, there have been substantial strides in hardware and
technique development. The methodology has made the evolutionary adaptation from
a purely laboratory based technology to an automated, operational tool useful for
compiling a corrosion history, screening a set of corrosion inhibitors, or optimizing a
water quality regime for corrosion control.
The suitability of an EC methodology is dependent on the corrosion morphology;
EC corrosion measures may be inappropriate on surfaces subject to heavy pitting-type
corrosion (i.e., mild steel or cast iron). But for uniformly corroding surfaces such as
copper, solders, zinc, and brass, EC methods can often provide an accurate and nearly
