GB2118309A - Apparatus for monitoring loss of metal by corrosion - Google Patents

Abstract

Apparatus for monitoring the rate of loss of metal due to corrosion from a surface covered by an electrolyte, comprises a two electrode probe connected to fluctuating signal measuring apparatus. Each electrode of the probe comprises a plurality of elements, each of which elements is electrically isolated from its immediate neighbours. Alternate elements are electrically connected together to provide each of said two electrodes and the elements forming at least one of said electrodes are comprised of the same material as the surface to be monitored. The probe may be provided with heating or heat sintering means, and may be flush mounted in a surface to be monitored. The signal measured may be impedance, electrochemical noise or galvanic response. <IMAGE>

Description

SPECIFICATION Apparatus for monitoring the rate of loss of metal from a surface The present invention relates to apparatus for monitoring the rate of loss of metal from a surface covered by an electrolyte, and particularly a thin film electrolyte, due to corrosion.

It is known to monitor the rate of loss of metal from a surface covered by an electrolyte using a multi-electrode probe mounted in or near the surface to be monitored in conjunction with electrochemical noise or a.c. impedance measurement techniques. At least one of the electrodes is comprised of the same material as the surface to be monitored and accordingly behaves in substantially the same way as the surface as a whole. By monitoring the output from the electrodes using the aforementioned measurement techniques it is possible to determine the charge transfer resistance of the electrochemical cell formed at the surface of the corroding electrode and hence the rate of loss of metal from the surface of interest.

Conventional probes comprise either two or three electrodes, the former arrangement using two identical plane electrode elements, each about 1 to 5cm square and the latter arrangement comprising a concentric arrangement of three electrode elements, with a reference electrode located in the centre, the middle ring forming a test electrode, and the outer ring an auxiliary electrode.

Both of the above described arrangements suffer from the disadvantage that only a relatively small area of each electrode lies adjacent the other, which gives rise to edge effects and consequently inaccuracies in corrosion rate assessment. In addition, the three electrode probe introduces reference potential fluctuations and reference electrode noise errors.

It is an object of the present invention to provide monitoring apparatus in which the above mentioned problems are obviated or mitigated.

According to the present invention there is provided apparatus for monitoring the rate of loss of metal due to corrosion from a surface covered by an electrolyte, comprising a two electrode probe connected to fluctuating signal measuring apparatus, wherein each electrode of the probe comprises a plurality of elements each of which elements is electrically isolated from its immediate neighbours, but alternate elements of which are electrically connected together to provide each of said two electrodes and the elements forming at least one of said electrodes is comprised of the same material as the surface to be monitored.

The fluctuating signal measuring apparatus may comprise a.c. impedance measuring apparatus, electrochemical noise measuring apparatus, or galvanic response to measuring apparatus.

Preferably the probe is flush mounted in the surface which it is desired to monitor the rate of loss of metal from.

Preferably the elements forming at least one electrode of the probe are obtained from the surface to be monitored.

Preferably the probe is provided with heating means and/or heat sinking means so that the temperature of the probe can be raised above or below ambient temperature.

Preferably the probe is provided with temperature sensing means, for example a thermistor.

Preferably the working surface of the probe is machined to match the contour of the surface in which it is to be mounted.

Preferably means are provided whereby external switching between elements comprising the electrodes can be achieved to permit study of localized corrosion phenomena.

Preferably means are provided whereby groups of elements can be isolated from each other for simultaneous connection to different fluctuating signal measuring apparatus.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which shouts a flush mounted probe for use in the present invention.

Referring to the accompanying drawing there is shown a corrosion monitoring probe for use in corrosion monitoring apparatus according to the present invention. The probe 1 comprises a plurality of plates 2 each electrically isolated from its immediate neighbours by insulation 3. Alternate plates are electrically connected together to provide a multi-element two electrode transducer. The elements may all be comprised of the same material as the surface it is desired to monitor or alternatively the elements forming each electrode may be of dissimilar materials one of which is of the same material as the surface to be monitored. The transducer assembly may be embedded or mounted in a non-metalic encapsulent electrically, mechanically and chemically appropriate to the conditions under which the probe is to be used.The working face 4 of the transducer or probe, that is the face subjected to corrosion, may be machined to match the contour of the surface within which the probe is to be mounted so that the working surface will be exposed to substantially identical conditions to that of the surface to be monitored.

In use the two electrode probe 1 is connected across fluctuating signal measuring apparatus, e.g., a.c.

impedance measuring apparatus, electrochemical noise measuring apparatus or galvanic response measuring apparatus. By galvanic response systems we mean the use of apparatus commonly known as Zero Resistarice or Null Impedance Ammeters to measure the galvanic current induced between the probes by the small potential fluctuations found to occur in corrosion processors. Analysis of the data in terms of absolute current gives information relating to the type of corrosion mechanism and in the limiting case the corrosion current.

Should it be required to monitor the corrosion rate at temperatures either above or below ambient temperature then the probe may be provided with a heating supply and/or heat sinking means. So that the temperature of the probe can be monitored a th.^rmistor o otnertemperature sensing device may be embedded immediately adjacent the plates, just r eFlind the wording face. The provision of heat sN!ty means is of particular advantage should it be required to cool the working face of the transducer to produce a condensed electrolyte fiim on the working surface thereof.

By placing successive electrodes in close arrangement to each other, the effects of ohmic error (the solution resistance of highly resistive corrosion electrolyte) can be minimised, which is particularly advantageous in high resistance thin fiim electrolytes. The precise spacing between plates is not critical, however clearly the plates should be as close as possible to keep to a minimum ohmic error. The spacing between electrodes may be widened however, where the transducer is to be used highly corrosive environments to reduce the likelihood of bridging of the elements by corrosion products or other debris.

The actual number of plates chosen for each electrode may be varied depending on the reliability of the results required. The more plates used the greater the stabiiity of the transducer and hence the results derived therefrom. Highly satisfactory results have been achieved however, using seven plates per electrode.

Where desired external switching of the plates may be provided so as to permit a study of the progressive corrosion behaviour across the working face of the probe, or the examination of highly localized corrosion phenomena. Although reducing the number of plates per electrode reduces the stability of the transducer this facility may still be highly useful in the laboratory or on site.

Moreover, by producing appropriate external switching of the plates it is possible to divide the probe into a number of sub-probes each of which may be used simultaneously connected to different measuring apparatus.

As stated hereinabove at least one of the electrodes is comprised of the same material as that of the surface to be monitored and indeed, where it is desirable that the corrosion information derived from the transducer corresponds almost exactly with the corrosion rate of the surface itself the electrode plates may be trepanned from the surface itself.

The monitoring apparatus according to the present invention premits the accurate determination of metal loss rates impossible to accurately monitor using currently availabie apparatus. For example corrosion of metal under crude oil or other organic substances such as ethanol or mineral oil is often due to minute quantities of water absorbed in the substance, but owing to the high electrical resistance of the substance accurate determination of the corrosion rate has been impossible.

Owing to the high stability of response derived from the probe of the present invention it is possible using the present invention to determine the rate of corrosion in such an environment.

The probe and measuring apparatus of the present invention can also be used to advantage to determine the rate of corrosion of metal in concrete at any point within the concrete where a probe has been installed. The rate of corrosion of metal within the concrete also indicates the change in the passivation behaviour of the concrete and therefore +he apparatus of the present invention can provide valuable information as to the integrity of the concrete, as well as to the corrosion rate of metal contained therein.

It will be appreciated that whilst the probe of the present invention has been described mounted in the surface from which the rate of loss of metal is to be monitored the probe may be mounted away from, but in the vicinity of, the surface of interest.

This would be the case where, for example, it was desired to monitor the rate of loss of metal from metallic packing material in a packed column. Usually the packing material comprises a plurality of metal tubular elements and by providing a probe in the form of a rod it is possible to insert the probe into the heart of the tubular elements where it is possible to monitor the rate of loss of metal for the tubular elements as a whole.

In a probe of the type described above the electrode may be in the form of metal washers, or hollow cylinders, as may be preferred, separated from each other by insulating gaskets and assembled together over a central support rod or screw fitted to each other.

Finally, it will be appreciated that the probe of the present invention may be adapted to represent a surface, the corrosion rate of which it is desired to monitor. For example, the probe may be formed with an aperture running through its longitudinal axis so as to represent a section of pipe. The probe may then be inserted in place of a section of pipe so as to monitor the rate of corrosion of a piece of pipe in that position, both externally and internally.

Claims (9)

1. Apparatus for monitoring the rate of loss of metal due to corrosion from a surface covered by an electrolyte, comprising a two electrode probe connected to fluctuating signal measuring apparatus wherein each electrode of the probe comprises a plurality of elements, each of which elements is electrically isolated from its immediate neighbours, but alternate elements of which are electrically connected together to provide each of said two electrodes and the elements forming at least one of said electrodes are comprised of the same material as the surface to be monitored.

2. Apparatus according to claim 1 wherein the fluctuating signal measuring apparatus comprises a.c. impedance measuring apparatus, electrochemical noise measuring apparatus, a galvanic response measuring apparatus.

3. Apparatus according to claim 1 or 2, wherein the probe is flush mounted in the surface which it is desired to monitor the rate of loss of metal from.

4. Apparatus according to claim 1, 2 or 3 wherein the elements forming at least one electrode of the probe are obtained from the surface to be monitored.

5. Apparatus according to any preceding claim, characterised in that the probe is provided with heating means and/or heat sinking means so that the temperature of the probe can be raised above or lowered below ambient temperature.

6. Apparatus according to claim 5, wherein the probe is provided with temperature sensing means, for example, a thermistor.

7. Apparatus according to any preceding claim, wherein means are provided whereby external switching between elements comprising the electrodes can be achieved to permit study of localized corrosion phenomena.

8. Apparatus according to any preceding claim wherein means are provided whereby groups of elements can be isolated from each other for simultaneous connection to different a.c. signal measuring apparatus.

9. Apparatus for monitoring the rate of loss of metal from a surface covered by an electrolyte substantially as hereinbefore described with reference to the accompanying drawing.