GB2039049A - Process and arrangement for the inspection and measurement of corrosion - Google Patents

Abstract

This invention relates to a process and arrangement for the inspection and measurement of the tendency to corrode of steel cast in concrete constructions or of freely exposed steel in metallic contact with such cast-in steel. One or more measuring probes cast in a steel-reinforced block (9) of concrete are employed. The probe or probes are capable of emitting electrical signals depending upon the parameters of the material in the environment of the probe. The electrical signals are conveyed via electrical conductor leads (7) to a measuring instrument for evaluation. Each probe comprises a measuring electrode 1 separated by insulating material 3 from a sleeve 2 of metal with stable electrochemical properties. The sleeves may be connected by steel plate 8 and insulated from associated reference electrodes 4. <IMAGE>

Description

SPECIFICATION Method and device for monitoring corrosion risk at steel structures in water-submerged concrete This invention relates to a process for the inspection and measurement of the tendency to corrode of steel cast in concrete constructions or of freely exposed steel in metallic contact with such cast-in steel. In addition, the invention includes a probe arrangement for use in carrying out the process.

Reinforced and prestressed concrete are used on a steadily increasing scale in both permanent and floating constructions in the marine environment. Gradually as the constructions become larger and are arranged in steadily deeper and more turbulent regions of the ocean, thedemands also increase for safety and functional efficiency. Experience has shown that with constructions in such an environment, there is both corrosion of the cast-in steel and particularly corrosion of the freely exposed steel in metallic contact with the cast-in steel which can create operational and maintenance problems.

Where integrated and connected components of steel come into metallic contact with the reinforcing system, an increased corrosion stress occurs on the freely exposed steel which depends upon both the access of oxygen into the cast-in steel and the surface area relationship between the cast-in and the freely exposed steel.

For large offshore concrete platforms where there are a series of freely exposed components of steel such as skirts, fastening means and pipes which often have a very small area relative to the area of the cast-in steel, the increased corrosion stresses can become very extreme. The criteria for cathodic protection also become very complicated.

Until a few years back, the entire assessment and control of reinforcement corrosion in concrete constructions was wholly based on visual inspection. A reinforcement corrosion does not make its appearance, however, before an advanced stage in the breakdown process which above water is characterised by rust specks, cracking up and scaling off of the concrete. At this stage, experience shows that it can be difficult to get the corrosion process under control. Under water, the corrosion process can seldom be discovered visually on the concrete surface.

However, it should be noted that the transition to active potentials is only one of the many factors required in order that the steel shall corrode. Even further information about the electrolytic resistance of the concrete provides no information of the speed of the corrosion.

A further disadvantage of the known processes is due to the fact that they are based on the surfaces of the constructions being generally accessible for control. Under water, this can create big problems, especially if the surfaces of the concrete are at the same time covered by heavy overgrowth.

There exists, therefore, a great need for a more complete and simplified process for inspecting and controlling the risk of corrosion, for example, based on probes which can be cast or moulded into the concrete at critical, selected locations of the construction and which can provide more or less continuous and complete information on potential, electrolytic resistance and speed of corrosion. At the same time, it is important to measure the cathode efficiency on the cast-in steel in order to be able to assess the increased corrosion stress on the freely exposed steel.

What is needed, therefore, is the provision of such an improved process as well as the necessary equipment for carrying out the process.

According to the present invention a process for the inspection and measurement of the tendency to corrode of steel cast in concrete constructions or of freely exposed steel in metallic contact with such cast-in steel, comprises utilising at least one measuring probe cast in a steelreinforced block of concrete and capable of emitting electrical signals depending upon the parameters of the material in the environrnent of said probe, the latter having its active measuring electrode electrically insulated from said steel reinforcement, and conveying electrical signals emitted by said probe to measure means for the evaluation of said signals.

The invention also includes a probe arrangement for carrying out the process of the last preceding paragraph and which comprises a steel member of the same kind of material as the steel reinforcement which is to be inspected, an insulating member in which said steel member is cast and a metal sleeve surroundingly supporting said insulating member and made of a material having stable electrochemical properties, one side of said steel member being exposed to concrete within the steel-reinforced block thereof to form its active measuring electrode, and measuring means electrically connected to said steel member and said steel reinforcement.

In order that the invention can be more clearly understood, a preferred embodiment thereof will now be described, by way of example, with reference to the accompanying drawing which is a side view of a steel-reinforced concrete block in contact with ocean water and having cast therein three probe components.

Referring to the drawing, steel members 1, 1' and 1" are of the same quality of steel as steel reinforcement 6 and are cast into respective blocks 3, 3' and 3" of an insulating material so that one surface is exposed and thereby forms a measuring electrode. Each of the steel members 1, 1' and 1" is connected to measuring equipment (not shown) via its respective electrical conductor lead 7. Each block of insulating material with its cast-in steel member is surroundingly supported by sleeve or pipe 2, 2' and 2" respectively. The sleeves are of more refined quality than the measuring electrodes and are secured to a common steel plate 8 of the same quality of material as the surrounding sleeve so that the three steel members lie by the side of one another.

A reference electrode 4 is secured parallel to the steel plate 8 but is electrically insulated from the latter by means of an insulating material 5. The steel reinforcement, the reference electrode and the steel plate are also connected to the measuring equipment via shielded conductor leads 7. The sleeves 2, 2" and 2" have different lengths so that the three electrodes of members 1, 1' and 1" are disposed at different and increasing distances from the steel plate 8. The steel plate 8 is secured parallel on its side to the steel reinforcement 6 closely up to the latter but is electrically insulated therefrom by insulating material 5.

Such a probe arrangement is located cast in the surrounding concrete 9 at critical positions in the concrete construction and is coupled to a suitable instrument system for inspection and control.

By means of this system the following measurements can be made: 1) By measuring the potential difference between the various electrodes and the reference electrode, the moment of the change of the potential of the steel from the passive to the active condition can be registered at different levels below the surface of the concrete.

In this connection, it shall be emphasised that the number of steel members 1, 1' and 1" and the mutual distances which are shown for the levels of their electrodes in the drawing, are only selected to illustrate the invention. It will be realised that the number of steel members and the differences in level can obviously be varied depending upon the field of use, the quality of the concrete and the risk of attack.

2) The electrolytic resistance from one cast-in steel member to another can also be measured. In addition to giving information of the importance of the steel corrosion, the electrolytic resistance also gives information concerning the degree of water saturation of the concrete.

3) By measuring the polarisation resistance of the electrodes, the speed of corrosion of the electrodes can be calculated. As a starting point, the speed of corrosion of a metal in an electrolyte is given by the following formula: hE K E = - (AE0) Al Ic where Al = the current which is necessary to change the potential AE.

It = the corrosion current K = the constant given for a given metal and a given electrolyte, determined, for example, by laboratory tests.

Measurements thus take place by supplying small potential, for example, less than 10 mV, and at the same time measuring the increase in current hI.

It should be mentioned that the speed of corrosion can only be measured on separate electrodes and not on the steel reinforcement itself. The reason for this is that the surface area of the steel which corrodes must be known. By using electrodes having steel of the same quality as the steel reinforcement, however, there is achieved a sound basis for assessing the state of corrosion and the speed of the steel reinforement's own corrosion.

In a simplified embodiment of the inventidn, only one probe component is used, the potential difference and the resistance between the one electrode and the reference electrode being measured.

4) By applying a negative potential on the electrodes and registering the stable polarisation current, the cathode efficiency for the electrodes at this potential is found. By selecting either the corrosion potential for the cast-in steel or the protection potential for cathodically protected cast-in steel, there is obtained a sound basis for assessing the cathode efficiency of the reinforcement under the relevant circumstances.

Since it takes a certain time before the current becomes stable, a stable reference electrode must be employed. In the drawing, there will be observed an external reference electrode 10 disposed in the sea.

Claims (8)

1. A process for the inspection and measurement of the tendency to corrode of steel cast in concrete constructions or of freely exposed steel in metallic contact with such cast-in steel and which comprises utilising at least one measuring probe cast in a steel-reinforced block of concrete and capable of emitting electrical signals depending upon the parameters of the material in the environment of said probe, the latter having its active measuring electrode electrically insulated from said steel reinforcement, and conveying electrical signals emitted by said probe to measuring means for the evaluation of said signals.

2. A process according to claim 1, wherein the potential difference between the active electrode and a reference electrode is measured to inspect the ability of the surrounding concrete material to passivate the steel reinforcement.

3. A process according to claim 1 or 2, wherein the electrolytic resistance between electrodes of two or more probes which are mutually spaced is measured.

4. A process according to claim 1, 2 or 3, wherein the current increase is measured through electrodes for a small increase in potential across the electrodes thereby establishing the polarisation resistance as an expression of the speed of corrosion.

5. A process according to claim 1, 2, 3 or 4, wherein the current through the electrodes at a given potential change is measured relative to an external reference electrode as a basis for calculating the penetration of oxygen into the concrete whereby an expression for the cathode efficiency of the steel reinforcement is found.

6. A probe arrangement suitable for carrying out the process of any of the preceding claims and which comprises a steel member of the same kind of material as the steel reinforcement which is to be inspected, an insulating member in which said steel member is cast and a metal sleeve surroundingly supporting said insulating member and made of a material having stable electrochemical properties, one side of said steel member being exposed to concrete within the steel-reinforced block thereof to form its active measuring electrode, and measuring means electrically connected to said steel member and said steel reinforcement.

7. An arrangement according to claim 6, wherein at least two steel members are so dimensioned that their associated electrodes are held at mutually different distances from a surface of the concrete.

8. Probe arrangements as claimed in claim 6 and substantially as described herein with particular reference to the accompanying drawing.