GB1263219A - Corrosion ratemeter - Google Patents

Abstract

1,263,219. Measuring corrosion rates. PETROLITE CORP. June 9, 1970, No.27996/70. Heading G1N. A corrosion rate meter of the type having three identical electrodes of the same material (e.g. mild steel) as the structure being monitored in which the current passed between one pair of the electrodes to produce a given polarization potential between another pair of the electrodes is measured, is provided with automatically operating nulling circuitry to compensate for the effects of the freely corroding potential and the resistive losses in the leads. In Fig.2, the electrodes are connected to terminals R, T and A. A sequencer connects in turn each of a plurality of testing stations and a meter prover to the circuit of Fig. 2. When a particular station is so connected, switches 67, 71, 89, 112 are first set to their "null" positions. The freely corroding potential plus IR lead potentials at R and T are connected to an isolation amplifier 36, the output of which is combined at 86 with a reference signal (which may be positive or negative) representing the polarization potential to be established between the electrodes connected to R and T and fed back via circuit 66 to input 38 of the isolation amplifier 36 to develop at equilibrium a corrective potential across resistor 47 equal to the algebraic sum of said freely corroding potential, IR lead potentials and polarization potential. At equilibrium the recorder displays the freely corroding potential. The switches 67, 71, 89, 112 are then changed over to their "operate" positions. Capacitor 81 stores a function of the corrective signal to maintain the potential across resistor 47. A current is caused to flow between the electrodes connected to A and T, the circuit reaching equilibrium when the said polarization potential has been established between the electrodes connected to R and T. The current through A and T is measured and indicated on meter 106. During this operation, the IR potentials at R and T remain unchanged. The "null" and "operate" sequence may be carried out twice, using the alternative positions of switches 87, 94, 118 to provide anodic and cathodic polarization potentials respectively. Adjustments. Potentiometer 41 is set to give zero output from the amplifier 36 when no electrodes are connected to the terminals. Switch 133 enables high or low gain to be selected in the "null" feedback circuit. Resistors 69, 134 and switch 136 enable range selection for the current between A and T. Meter prover. This is a resistance network which simulates a known corrosion rate when it is connected to the circuit of Fig. 2, by the sequencer. If all is well, the recorder indicates during the "operating" phase the current between A and T appropriate to this known corrosion rate.