Low Cost Corrosion Monitoring Sensors and System

The Analatom Inc. micro-machined LPR corrosion sensor is part of the company's structural health monitoring system.  The sensor is built on well-established LPR (Linear Polarization Resistor technology) and it outputs an exact corrosion rate for the structure on which it is placed.  The device is fabricated using semiconductor production techniques that allow for a high quality product that is low cost and robust.  Analatom has relationships with many materials suppliers ensuring that sensors are available for most metals.

Applications

The sensor can be placed on bare metal and all standard coating applied.  This allows for fitting to virtually any structure.  Typical applications are:

• Aerospace
• Bridges
• Pipeline
• Automotive
• Air Conditioning systems

Sensors are connected to a central data node that processes the sensors output, stores the data and can communicate the data via wireless, Bus or serial formats.  The node is also capable of recording data from most standard off the shelf sensors.  Nodes can be battery powered and have a typical life of 7-years between battery changes when used for corrosion monitoring.  Solar and bus powered systems are also available.

Figure 1: Typical corrosion on a 1010 steel LPR

Technical Brief

The electrochemical technique commonly referred to as Linear Polarization Resistance (LPR) allows for measurement of rates of corrosion directly, in real time. The polarization resistance of the material is defined as the slope of the potential-current curve at the free corrosion potential. The rate of corrosion is determined by the change in the properties of the metal-environment interface due to reactions taking place on the metal surface, which influence the corrosion current density.
Corrosion occurs when a metal or alloy is exposed to any fluid of sufficient oxidizing power.  In Figure 1 we see a corroding device that gives good signal levels. At the interface, metal ions will escape from the metal surface leaving a surplus of electrons. This excess electron flow, from the anodic sites on the metal surface (where they are generated) to cathodic sites, where they are consumed, constitutes a corrosion current.  This current is a measure of the loss of the metal from the surface. The corrosion current (Icorr) can be calculated from the linear polarization resistance and then used to estimate the corrosion rate. As the anodic and cathodic sites continually shift and change their positions, Icorr cannot be directly measured from the metal surface. A small potential drop (ΔE) has to be applied externally to induce a measurable current flow (ΔI) at the corroding surface. At given values of ΔE, Icorr is directly proportional to the induced current ΔI and the relationship is given as:

                         (Equation 1)

Where, βa and βb are the Tafel constants.  The Analatom system shown in Figure 3, is effectively a multiplexed potentiostat is used to change the potential on the metal surface in a controlled manner so that the corresponding current values can be measured as a function of the potential. The relationship between ΔE and ΔI is linear at values of ΔE close to that of the equilibrium potential (Eo), assumed by the metal in absence of any induced potential ΔE. The slope of this line has the value ΔE/ ΔI and has the units of resistance. The slope is therefore called polarization resistance, Rp. The value of Rp obtained from a potential sweep over a range can then be used to determine Icorr, as shown by the relation above. The Tafel constants can be obtained from Tafel plots of the system under consideration. Furthermore, the rate of corrosion can be calculated from the corrosion current by using Faradays relation that correlates current flow to mass loss, given as,

                        (Equation 2)

Where; CR = corrosion rate, EW = equivalent weight of the material in grams/ equivalent, k = constant, d= density, A= sample area.

 
Figure 2: Analatom Sensor Node

Specification

Figure 3 : Coated / Uncoated Analatom LPR sensor.