Patent GB-1369191-A (GB)
Title: Corrosion ratemeter
IPC: G01N27/26
Filed: 1973-07-10
Abstract: 1369191 Measuring corrosion PETROLITE CORP 10 July 1973 [6 Oct 1972] 32892/73 Heading G1N A corrosion rate-meter of the kind in which the current applied between a test specimen electrode 21 and an auxiliary electrode 23 to produce a predetermined polarization potential between the test and a reference electrode 22 is measured includes a circuit arrangement to compensate for the freely corroding potentials present due to the half cells formed by the electrodes in the corrodant. The reference 22 and test 21 electrodes are connected to inputs 31, 32 of a high gain, high input impedance differential input amplifier 33, electrode 21 via a potential producing circuit 36. Any potential difference across inputs 31, 32 results in a current flow in a current loop from the output 39 to the auxiliary electrode 23, through the corrodant to test electrode 21 and thence via lead 29 to input 47 of a second differential amplifier 46, meter 54 and to circuit common CM. The second amplifier input 49 is also connected to circuit common and the circuit functions such that circuit common is brought to zero potential difference relative to the test electrode. The potential producing bridge circuit 36 supplied from battery 76 is such that at balance the potential at top centre tap 69 connected to input 32 and at adjustable tap 73 connected to test electrode 21 are the same. With the switches 66, 67 in the nulling position Z resistor 73 is adjusted until meter 54 reads zero, i.e. a potential is applied to terminal 32 to correspond to the freely corroding potential produced across inputs 31, 32 from the test and reference electrodes and effectively removing this potential. Switches 66, 67 are then placed in either cathodic or anodic positions C, A so that resistors 68 and 71 are incorporated into the bridge to unbalance it by a predetermined amount e.g. 10 m V, and of the appropriate polarity, fine adjustment of potential being made by resistor 84. This second potential signal is applied to the inputs of amplifier 33 and is of the necessary magnitude to produce a current flow in the output 39 so that the test electrode reaches the desired polarization potential relative to the reference electrode 22. The current flow is measured at 54 to provide the corrosion rate. A feedback circuit connected across amplifier 46 from output 51 to input 47 and comprising a switched resistor chain 57, 58a-j provides for a plurality of ranges. Measurement of freely corroding potential:- Switch 78 is moved to the NONE position so that no potential signals are applied from network 36 to the input of amplifier 33 and switches 41, 48 and 52 are moved to one of the millivolt positions. The freely corroding potential across inputs 31, 32 from the reference 22 and test electrodes results in an output current at 39 which flows via calibration resistor 53 and meter 54 to circuit common. The freely corroding potential is indicated at 54. Amplifier 46 functions in this mode to bring circuit common to the same potential as electrode 21 and to provide feedback across resistors 37 and 38 to the input circuit of amplifier 33 whereby the potential difference between inputs 31 and 32 is brought to zero and the corroding potential is measured without current flow from the electrodes.