Downhole corrosion monitoring

1. A system, comprising:

a downhole tool configured for conveyance within a borehole extending into a subterranean formation and comprising a metal portion exposed to a flow path for a downhole fluid; and

an electrochemical corrosion sensor comprising:

an electrical contact coupled to the metal portion;

an electrochemical cell housing a reference fluid substantially similar to the downhole fluid;

a reference electrode disposed in the reference fluid; and

an electronics unit coupled to the electrical contact and the reference electrode to monitor, based on a measured electrical potential difference, corrosiveness of the downhole fluid.

2. The system of claim 1 wherein the reference electrode comprises a material similar to that of metal portion.

3. The system of claim 1 wherein the downhole fluid comprises at least one of a drilling fluid, a completion fluid, an injection fluid, and a subterranean formation fluid.

4. The system of claim 1 wherein the metal portion is part of a wireline cable.

5. The system of claim 1 wherein the metal portion is part of a wired drill pipe.

6. The system of claim 1 wherein the sensor is disposed within a cavity of a tool body of the downhole tool, wherein the cavity comprises a piston disposed therein and configured to pump the fluid within the cavity.

7. The system of claim 1 comprising a telemetry unit coupled to the sensor to transmit an indicator of the corrosiveness to a surface unit.

8. The system of claim 1 comprising a control system configured to receive an indication of the corrosiveness from the sensor and to generate an alert in response to determining that the corrosiveness is outside a desired range.

9. The system of claim 1 comprising a control system configured to analyze the corrosiveness and recommend a change in composition of the downhole fluid based on the analysis.

10. The system of claim 1 , wherein the metal portion comprises a flow line configured to direct formation fluid into the downhole tool from the subterranean formation.

11. A system comprising:

a downhole tool configured for conveyance within a borehole extending into a subterranean formation and comprising a metal body defining an internal flow path through the downhole tool for a drilling fluid; and

a first electrochemical corrosion sensor comprising:

an electrical contact coupled to the metal body;

an electrochemical cell housing a reference fluid substantially similar to a downhole fluid;

a reference electrode disposed in the reference fluid; and

an electronics unit coupled to the electrical contact and the reference electrode to monitor, based on a measured electrical potential difference, a first corrosiveness of the drilling fluid flowing through the internal flow path.

12. The system of claim 11 comprising a second electrochemical corrosion sensor coupled to an outer body of the downhole tool to monitor a second corrosiveness of the drilling fluid flowing past the outer body of the downhole tool.

13. The system of claim 12 comprising a control system configured to compare the first corrosiveness to the second corrosiveness to determine whether a formation fluid is entering the borehole from the subterranean formation.

14. The system of claim 12 , wherein the metal body comprises a chassis and wherein the outer body comprises a drill collar.

15. The system of 11 , wherein the first electrochemical corrosion sensor comprises a heat sink coupled to the metal body and the electrochemical cell.

16. The system of claim 11 , comprising a control system configured to receive an indication of the corrosiveness from the first electrochemical corrosion sensor and to generate an alert in response to determining that the corrosiveness is outside a desired range.