CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase entry, under 35 U.S.C. Section 371(c), of International Application No. PCT/EP2018/055475, filed Mar. 6, 2018, claiming priority from European Patent Application No. 17159834.5, filed Mar. 8, 2017. The disclosures of the International Application and the European Application from which this application claims priority are incorporated herein by reference in their entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND

The present invention relates to a method of detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, and to a system for detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment. The present invention also relates to a probe for detecting locally generated corrosion of a metal element when exposing said metal element to a corrosive environment.

Corrosion is a process, which converts a refined metal such as steel to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of the steel by a chemical and/or an electrochemical reaction with the environment. A corrosive environment typically comprises water and ions, which constitute an electrolyte solution that can carry charges and conduct current. Especially under water or under ground the environment can be seen as an electrolyte solution which conducts electric charges and chemically reacts with metal. In general, the more ions there are in the electrolyte solution the more conductive the electrolyte solution becomes.

As the metal corrodes the resistance of the metal increases. The resistance of an element is a function of its length, width, thickness and the resistivity of the metal that the element is made of. The resistance is inversely proportional to the thickness of the element, which means that the resistance increases as the thickness decreases due to corrosion.

U.S. Pat. No. 6,936,158 describes a system where the corrosion of an object exposed to a corrosive environment is implicitly monitored. A probe with a metal is placed proximate to the object. The resistance of the metal is then measured. U.S. Pat. No. 7,541,817 describes another system for implicitly monitoring the corrosion of an object exposed to a corrosive environment. U.S. Pat. Nos. 6,936,158 and 7,541,817 are incorporated in the present application by reference.

The above examples of systems for implicitly monitoring the corrosion of an object exposed to a corrosive environment are not suitable for monitoring pitting corrosion, or pitting, which is a form of extremely localized corrosion that leads to the creation of small holes in the metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.

SUMMARY

The above object and advantages together with numerous other objects and advantages, which will be evident from the description of the present invention, are according to a first aspect of the present invention obtained by:

a method of detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, comprising:providing said metal element constituting a first element defining opposite first and second surfaces, said first surface defining an exposed outer surface to be exposed to said corrosive environment, said first element comprising a first pair of terminals,providing a first pair of electrical connectors, and contacting each connector of said first pair of electrical connectors with a respective terminal of said first pair of terminals for allowing an injection of an electric current into said first element,providing a sponge, said sponge constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in said corrosive environment, and applying said sponge to said second surface of said first element,providing a second element of an electrically conductive material, and arranging said second element opposite to said second surface of said first element, and sandwiching said sponge between said first element and said second element,providing an electrical insulator, said electrical insulator together with said first element encapsulates said second element and said sponge for preventing said second element and said sponge from being directly exposed to said corrosive environment,injecting a specific electric current into said first element via said first pair of electrical connectors, and measuring a voltage between the terminals of said first pair of terminals, and monitoring a gradual increase in said voltage due to a corrosion of said first element, anddetecting a decrease in said voltage caused by a pinhole being generated in said first element, causing said sponge to be wetted and allowing said second element to provide a shunt relative to said first element thereby reducing a resistance between the terminals of said first pair of terminals.

According to a second aspect of the present invention, the above objects and advantages are obtained by:

a method of detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, comprising:providing said metal element constituting a first element defining opposite first and second surfaces, said first surface defining an exposed outer surface to be exposed to said corrosive environment, said first element comprising a first pair of terminals,providing a first pair of electrical connectors, and contacting each connector of said first pair of electrical connectors with a respective terminal of said first pair of terminals for allowing an injection of an electric current into said first element,providing a sponge, said sponge constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in said corrosive environment, and applying said sponge to said second surface of said first element,providing a second element of an electrically conductive material, and arranging said second element opposite to said second surface of said first element, and sandwiching said sponge between said first element and said second element,providing an electrical insulator, said electrical insulator together with said first element encapsulates said second element and said sponge for preventing said second element and said sponge from being directly exposed to said corrosive environment,measuring an impedance between said second element and a ground electrode, andmonitoring a decrease in said impedance from an infinite impedance to a finite impedance caused by a pinhole being generated in said first element, causing said sponge to be wetted and an electric conducting path being established between said second element and said ground electrode.

The sponge may be made of a laboratory grade filter paper so that the wetting of the sponge does not differ with more than 20% from one sponge in one probe to another sponge in another probe. In this way, the output data from a probe becomes more consistent between probes. When the probe is used underwater, it is desirable to use a sponge rather than air, because the sponge is more robust against high pressure.

The sponge and the second element are on top of each other and constitute a two layer structure. The sponge separates the second element from the first element. The sponge and the second element need to be protected from the environment around the probe at least until a pinhole is in the first element. The first element may be placed on top of the sponge so that the second surface faces the sponge. In this way the second element and the sponge are at one side encapsulated or shielded from the environment. At the remaining sides, a housing may encapsulate or shield the second element and the sponge. The housing may comprise an opening or a window, in which the first element is placed with the first surface facing the surrounding environment/corrosive environment. In this way, the first surface of the first element is directly exposed to the corrosive environment while the second element is indirectly exposed and becomes directly exposed when a pinhole is in the first element. Alternatively, the second element and the sponge may be encapsulated at the remaining sides by an epoxy.

The first pair of terminals is constituted by a first terminal and a second terminal. A terminal may be an area on the first element on which a connector is soldered or connected to the first element. Preferably, the first terminal is at a first end of the first element and the second terminal is at a second end of the first element. The first end and the second end being opposite.

The first pair of electrical connectors is constituted by a first connector and a second connector. The first connector may be connected to the first terminal and the second connector may be connected to the second terminal.

For injecting a current in the first element, the first pair of electrical connectors establishes an electric circuit path in which electrons may move, i.e. the first connector is a connection from a first current generator to the first terminal, and the second connector is a return path from the second terminal to the first current generator. To provide a stable current through the first element a first resistor with a high resistor value such as 10 kΩ or more may be connected in series with the first element. The current may be an alternating current (AC current) of a frequency between 0.5 kHz and 10 kHz.

The voltage across the first element may be measured by a first analyzing unit, wherein a voltmeter function is implemented. A first pair of wires constituted by a first wire and a second wire may connect the first terminal and the second terminal respectively with the first analyzing unit.

A specific electric current means a current of a specific value such as 10 mA or less, which does not change with the voltage across the first element (the voltage drop or simply voltage between the first terminal and the second terminal). This makes the current a known parameter and with a measured value of the voltage across the first element, the resistance of the first element can be determined from Ohm's law.

The resistance of the first element depends primarily on the material it is made of and its shape. The thickness of the first element may be derived from the resistance derived from the current and the voltage. As the voltage is monitored as a function of time, the thickness can be monitored as a function of time. The resistance or the thickness of the first element may be compared to a first threshold value, i.e. if the thickness is 50% less than the initial thickness an alarm may be set or if the resistance is 50% greater than the initial resistance an alarm may be set.

The corrosion of the first element may result in a pinhole in the first element, i.e. a through hole in the first element. If that happens, the encapsulation of the sponge and the second element is no longer complete and an electrolyte solution in the corrosive environment may flow through the pinhole and wet the sponge making the sponge and second element a shunt with respect to the first element. This effectively decreases the resistance between the first terminal and the second terminal, which means that the voltage across the first element decreases, since the current is held constant. The first analyzing unit may compare the voltage with a threshold value, i.e. if the voltage has decreased with 50% an alarm is set, the comparison may be conditioned upon a time window, i.e. a flag is set if the voltage has decreased with 20% within a period of 10 or 5 minutes.

The second element may comprise a back terminal, wherein a second element wire is provided and the back terminal is contacted by the second element wire for measuring the impedance between the second element and the ground electrode. The second element wire may connect the back terminal with the second analyzing unit wherein an impedance measuring function is implemented. The first analyzing unit and the second analyzing unit may be an integral unit.

The impedance may be a result of an induced current in the electrolyte solution in the ground or water due to a nearby powerline. Thus, an AC current will flow in the electrolyte solution between the ground electrode and the second element. The second analyzing unit may determine the magnitude of the impedance and the phase difference between the induced AC current and voltage. Alternatively, a second current generator or a voltage source may be used to induce an AC current in the electrolyte solution such that a current may flow between the ground electrode and the second element.

The ground electrode may be connected to a ground, which may serve reasonably as a constant potential reference against which other potentials can be measured. In electronic circuit theory, a ground is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential/voltage.

Preferably, the ground electrode is an earth electrode placed into the earth proximate to the first element, which means that the ground electrode is not more than 10 m from the first element. If the first element is placed in water, the ground electrode may be inserted down into the seabed.

A third element of a metal for defining a temperature reference with respect to the first element may be provided. The third element being encapsulated for preventing exposure to said corrosive environment.

The third element may comprise a second pair of terminals, wherein a second pair of electrical connectors is provided and each connector of the second pair of electrical connectors is contacted with a respective terminal of the second pair of terminals for allowing an injection of an electric current into the third element.

A specific electric current may be injected into the third element via the second pair of electrical connectors, and a voltage between the terminals of the second pair of terminals may be measured

The voltage between the terminals of the first pair of terminals may be compared with the voltage between the terminals of the second pair of terminals for determining a voltage bias of the first element.

The first element may be provided with a first shape constituted by a first length, a first width, and a first thickness, and the second element may be provided with a second shape constituted by a second length, a second width, and a second thickness, wherein a difference between the first shape and the second shape is less than 20%.

The first thickness may be determined as a function of the specific electric current and the voltage between the terminals of the first pair of terminals.

According to a third aspect of the present invention, the above objects and advantages are obtained by:

a system for detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, said system comprising said metal element,wherein said metal element constituting a first element defining opposite first and second surfaces, said first surface defining an exposed outer surface to be exposed to a corrosive environment, said first element comprising a first pair of terminals,said system further comprising a first pair of electrical connectors, wherein each connector of said first pair of electrical connectors is connected to a respective terminal of said first pair of terminals for allowing an injection of an electric current into said first element,a sponge, said sponge constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in said corrosive environment,wherein said sponge is juxtaposed said second surface of said first element,a second element of an electrically conductive material,wherein said second element is opposite to said second surface of said first element and wherein said sponge is sandwiched between said first element and said second element,an electrical insulator, said electrical insulator together with said first element encapsulates said second element and said sponge for preventing said second element and said sponge from being directly exposed to said corrosive environment,a current generator for injecting a specific electric current into said first element via said first pair of electrical connectors,a voltmeter for measuring a voltage between the terminals of said first pair of terminals, and monitoring the gradual increase in said voltage due to the corrosion of said first element, anda first analysing unit for detecting the decrease in said voltage caused by a pinhole being generated in said first element, causing said sponge to be wetted and allowing said second element to provide a shunt relative to said first element thereby reducing the resistance between the terminals of said first pair of terminals.

According to a fourth aspect of the present invention, the above objects and advantages are obtained by:

a system for detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, said system comprising said metal element,wherein said metal element constituting a first element defining opposite first and second surfaces, said first surface defining an exposed outer surface to be exposed to a corrosive environment, said first element comprising a first pair of terminals,said system further comprising a first pair of electrical connectors, wherein each connector of said first pair of electrical connectors is connected to a respective terminal ofsaid first pair of terminals for allowing an injection of an electric current into said first element, a sponge, said sponge constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in said corrosive environment,wherein said sponge is juxtaposed said second surface of said first element,a second element of an electrically conductive material,wherein said second element is opposite to said second surface of said first element and wherein said sponge is sandwiched between said first element and said second element,an electrical insulator, said electrical insulator together with said first element encapsulates said second element and said sponge for preventing said second element and said sponge from being directly exposed to said corrosive environment,a current generator for injecting a specific electric current into said first element via said first pair of electrical connectors,a voltmeter for measuring a voltage between the terminals of said first pair of terminals, and monitoring a gradual increase in said voltage due to the corrosion of said first element, anda measuring unit for measuring an impedance between said second element and a ground electrode anda first analyzing unit for monitoring a decrease in said impedance from an infinite impedance to a finite impedance caused by a pinhole being generated in said first element, causing said sponge to be wetted and an electric conducting path being established between said second element and said ground electrode.

According to a fifth aspect of the present invention, the above objects and advantages are obtained by:

a probe for detecting a locally generated corrosion of a metal element when exposing said metal element to a corrosive environment, said probe comprising said metal element,wherein said metal element constituting a first element defining opposite first and second surfaces, said first surface defining an exposed outer surface to be exposed to a corrosive environment, said first element comprising a first pair of terminals for connecting to a first pair of electrical connectors,a sponge, said sponge constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in said corrosive environment,wherein said sponge is juxtaposed said second surface of said first element,a second element of an electrically conductive material,wherein said second element is opposite to said second surface of said first element and wherein said sponge is sandwiched between said first element and said second element,an electrical insulator, said electrical insulator together with said first element encapsulating said second element and said sponge for preventing said second element and said sponge from being directly exposed to said corrosive environment.

The probe may be used to derive an indication of a state of corrosion of an object such as a pipeline or a supporting structure underground or under water. Such objects are typically exposed to a corrosive environment because of the presence of water and ions, which constitute an electrolyte solution that can carry charges and conduct current.

The probe is preferably placed in proximity to the object of interest and the first element is preferably made of the same material as the object, typically steel. If information about the material properties of the object is not available, the first element may be made of a corrosive metal such as steel.

According to a fifth aspect of the present invention, the above objects and advantages are obtained by a probe according to the fourth aspect of the present invention used for deriving an indication of a corrosion of a pipeline or an underwater structure such as an oil platform leg.

