The present invention relates to a corrosion probe used in both polarization and weight loss measurements of corrosion rates of metallic material upon exposure to a corrodant electrolyte.
Initially, corrosion rates were determined by simply inspecting the surfaces of corroded equipment. Such a method is undesirable since the equipment is often irreparably damaged before corrosion is evident. Furthermore, it is often necessary to shut-down equipment in order to make an inspection.
Accordingly, small test specimens were developed for relatively simple insertion and removal from a representative part of a corroding system to be tested. Typically, such test specimens are integral parts of a corrosion probe assembly which additionally includes support equipment for the specimens. These probe assemblies are designed for ease of incorporation into the test system. For example, the assembly may be designed as a pipe fitting, requiring no special tools for insertion techniques.
Two types of corrosion probe assemblies commonly used are known as weight loss types and polarization resistance types. Measurement of corrosion rate by weight loss probe assemblies is performed by initially measuring and recording the test specimen's initial weight to give W.sub.i (grams). The probe assembly is installed in the corrodant system such that the specimen is subjected to the system fluid, such as cooling water. The time of installation is recorded. At the end of the test period the specimen is removed and carefully cleaned in order to remove all corrosion products without removing uncorroded specimen material. Also, the time of specimen removal is recorded. The test specimen's final weight is measured to give W.sub.f (grams). The corrosion rate is calculated from this data from: EQU Corrosion rate (mpy) = K (W.sub.i - W.sub.f)/t (1)
where t = time duration of the test (days) and K = constant factor related to the metal being tested, its density, and its surface area.
Polarization type probe assemblies make use of the electrochemical nature of corrosion. Such probes typically utilize test, reference, and auxiliary electrodes. The test electrode, in the form of a test specimen, in initially permitted to corrode freely. A small electric current is passed through the corrodant between the test and auxiliary electrodes while the polarization potential between the test and reference electrodes is measured. The current is increased until a small given change in electrode potential, usually 10 mv, is measured between the test and reference electrodes. The resulting current required to produce the change is proportional to the instantaneous corrosion rate. Accordingly, this resultant current can be read and converted into units of corrosion rate. In fact, one among the inventors, in U.S. Pat. No. 3,716,460, which is hereby incorporated by reference, discloses a device which gives direct readings of instantaneous corrosion rate from a polarization-type probe assembly.
There is a third type of corrosion probe commonly used based on a measurement of electrical resistance of a small wire preferably manufactured from the same metal as in the test system. As the wire corrodes, its resistance increases since the diameter gets smaller. Thus, the change of resistance with time is a measure of the corrosion rate of the wire. The main disadvantage is that localized corrosion (pitting, deposit corrosion, etc.) has a tendency to cause the wire to corrode in a small area. Thus, the resistance measured will be determined largely by the smaller diameter which exists at the localized corrosion site. Also, since this site corrodes much faster than the rest of the wire, there is a tendency for the wire to break off at this point. The prime advantage of this technique is its ability to measure metallic corrosion in nonelectrolyte systems.
It is a common observation among workers in this field that there is often a discrepancy between hot corrosion and scaling occurring on surfaces experiencing heat transfer (e.g., heat exchanger surfaces) and cold corrosion and scaling occurring on those not experiencing heat transfer (e.g., corrosion specimens). Accordingly, the inventors set out to develop means for accurate corrosion rate measurements either by polarization resistance or by weight loss techniques wherein heat transfer is simultaneously taking place.
While conducting this developmental work, the inventors demonstrated that an interrelationship exists between the heat transfer coefficient (U) and corrosion and scaling in a system. As deposition increases it was observed that U is reduced. Also, corrosion products can contribute to the amount of deposition. In addition, reduced flow rate of the system fluid reduces U. It thus occurred to the inventors that a determination of heat transfer coefficient (U) in a corrodant system wherein corrosion and/or deposition occurs would provide an additional important tool in evaluating the system. In U.S. Patent application Ser. No. 430,453, filed on Jan. 3, 1974, now U.S. Pat. No. 3,918,300, which is hereby incorporated by reference, a U-meter is disclosed which provides an instantaneous U reading in a heat transfer system.
The probe assembly of the present invention permits detection of the necessary corrodant system parameters and feeding of the detected parameters into the U-meter for an instantaneous read-out of U. A test electrode or a test specimen of the probe assembly is heated, preferably electrically heated. The power W (watts), can be directly converted into heat, Q (BTU/hour), from the following relation: EQU Q = 3,413 W (2)
the heat flux (Q/A) is, then, EQU flux = Q/A = 3.413 W/A (3)
where A = test material specimen surface area. If, for example, the test material surface area is 9 cm.sup.2 the flux would be as follows: EQU flux (BTU/hr - ft.sup.2) = 352.3 W (watts) (4)
If 10,000 BTU/hr - ft.sup.2 (a reasonably high amount of heat flux in any plant heat exchanger) where needed in the probe assembly, then from equation (4), 28.4 watts should be fed to the heater.
At a fixed flow rate, U is defined by the equation: EQU Q/A = U(T.sub.2 -T.sub.1) (5)
where Q, A and U have been previously described; T.sub.2 = temperature of test material surface (.degree.F); T.sub.1 = external fluid temperature (.degree.F). Thus, EQU U = (Q/A)/(T.sub.2 - T.sub.1) (6)
all that is now necessary to determine U is measurement of T.sub.2 and T.sub.1. This can be accomplished, for example, by placing a temperature probe in the fluid stream near the heat transfer surface to determine T.sub.1 and placing a temperature sensor within the test specimen to determine T.sub.2. It should be noted from equation (6) that, at constant Q/A (constant W), T.sub.2 - T.sub.1 is sufficient to completely define U. Thus, the direct measurement of T.sub.2 - T.sub.1 is sufficient to determine the heat transfer coefficient.
During their work, the inventors discovered yet another important tool in evaluating corrodant systems and the treatment thereof. By simultaneously measuring cold corrosion and hot corrosion, the effect of the heat transfer load on the corrodant system can be determined. This can be accomplished by providing the probe assembly of the present invention with both a hot test specimen and a cold test specimen.
In fact, by providing a probe assembly with plural test sepcimens, various and sundry tests can be run to evaluate a corrodant system treatment program. For example, the simultaneous effect of heat load and different metallic materials in a corrodant system could be readily examined. Also, for example, galvanic interaction between two specimens can be studied by electrically connecting them externally and measuring current flowing between them.
Accordingly, it is an object of the present invention to provide a novel corrosion probe assembly which is very versatile in evaluating a corrodant system treatment program.
It is a further object of the present invention to provide a corrosion probe assembly which facilitates measurement of corrosion rate under specified heat transfer conditions.
Yet a further object of the present invention is to provide a corrosion probe which facilitates measurement of heat transfer coefficient simultaneously with measurement of corrosion rate.
An additional object of the present invention is to provide a corrosion probe which facilitates simultaneous measurement of hot and cold corrosion .