Corrosion Coupons
Introduction:
When water is treated for industrial use, it is done primarily to alleviate or reduce the effects of scale and corrosion in heat exchangers or steam generating systems. The effects of scale are readily apparent through physical evidence. Scale can be seen, and its presence can be confirmed by differential temperature measurements at heat exchangers. In steam generating systems, reduced efficiency can indicate the presence of deposits on heat transfer surfaces.
But what about corrosion? Unfortunately, the first noticeable effects of corrosion usually appear after severe damage has taken place – the hot condensate dripping from a corroded pipe fitting, or the fluid contamination across a condenser or heat exchanger.
How can we monitor corrosion in order to get a reasonable feel for what is happening inside the equipment? Several methods are available for measuring corrosion. None are completely accurate, but do serve as useful tools for determining the corrosion rate and the type of corrosion, which is occurring. We will discuss two of the most frequently used methods.
Corrosion Strips or Coupons:
The two systems most susceptible to insidious corrosive attack are cooling water systems and steam condensate return systems. An easy method for determining corrosion within these systems is with the use of corrosion test coupons.
Coupons are weighed strips of metal that are suspended in a flowing water stream for a specified period. Corrosion rate is calculated from the weight loss during this period of exposure. Coupons are available in a choice of metals including aluminum, brass, copper, and steel. They are prepared in accordance with the ASTM standard procedure, and then fastened to an insulating holder, which, in turn, is fastened to a standard pipe plug.
Installation:
For determining the corrosion rate in open recirculating cooling systems, best results are obtained by installing a coupon rack on the cooling water return riser. Install test strip assemblies at each tee; being careful to keep the coupons free of dirt and grease, and installed in such a way that the strip does not contact the system metal. Adjust the flow rate through the rack to approximate the velocity desired. Optimum velocities for various metals are as follows:
Metal Range
Carbon steel 2.5-6.0 ft/sec Admiralty 2.5-5.0 ft/sec Aluminum brass 4.0-8.0 ft/sec
Coupons should be removed at intervals, beginning after 30 days; then 60 days and the third one removed after 90 days exposure. Record the number of days exposed, as this is an important factor in determining the actual corrosion rate.
High Surface Temperature Corrosion:
One limitation of the test strip or coupon method is the fact that the coupon temperature will be that of the water. Metal surface temperature in some systems, however, can be much higher than the bulk water temperature. Corrosion rates at high temperatures can be much greater than an unheated strip would indicate. In order to more accurately determine the corrosion rate in systems with high surface temperatures, several types of test heat exchangers can be employed. One type consists of a heater and circulating system to circulate hot fluid through a “U-tube” specimen, which is installed in the water system. Installation is similar to that of a standard coupon assembly. The circulating fluid inside the “U-tube” can be heated to a maximum temperature of 400°F. By this means, it is possible to measure the effect of high heat flux on the corrosion rate. U-tube specimens are weighed before and after the test, similar to the procedure for test strips.
Steam Condensate System:
Test strips are a valuable method of monitoring the corrosion rate in condensate return systems. However, the location of the test strip assembly must assure that the coupon is always submerged in condensate for accurate results. One frequently used location which meets this requirement is down stream from a condensate return pump. If possible, install testers at several locations in the system to help detect any uneven distribution of condensate treatments or excessive accumulations of carbon dioxide in the condensate.