May 1, 2009

How to Measure Corrosion Processes Faster and More Accurately

Daniel C. Sampson and Peter D. Hicks, Nalco

Establish an ORP Reference

ORP is the electrical potential of a liquid at a specific temperature. As such, ORP represents the net sum of redox potentials/reactions in a sample stream as measured on noble metals (usually platinum or gold). Measurements are usually made against some standard reference electrode such as silver/silver chloride or copper/copper sulfate. ORP is the difference in voltage between these two electrodes.

It’s important to quote the temperature and other conditions at which the reading was taken when an ORP number is quoted and the reference electrode is used to measure the potential difference. For example, consider an ORP sample taken at 400F when an ORP number is simply quoted as "–0.6 V." This value is essentially meaningless. There’s no way of knowing if the voltage measured is –0.6 V against the standard hydrogen electrode (SHE) at 400F, –0.6 V against the SHE at 77F, –0.6 V against some other reference electrode at 400F, or –0.6 V against some other reference electrode at 77F. The plant chemist should be aware of the importance of temperature when investigating and comparing competing ORP claims.

For the purposes of this article, we use the ORP value versus the reference electrode with temperature. For example, an ORP measurement of –0.6 V obtained with the SHE at 400F is reported as –0.6 V vs. SHE (400F).

At-Temperature ORP

The at-temperature (AT) ORP program continuously monitors the corrosion stress in a hot water system such as boiler feedwater. It provides a window into the process that’s not available from the traditional room-temperature measurements such as DO, ORP, or reductant residuals tests. The measurement also allows real-time analysis of the corrosion kinetic continuum rather than providing a postulated measurement that assumes thermodynamic stability and equilibrium, allowing operators to take rapid action to control and/or identify corrosion stress to minimize corrosion and subsequent corrosion product transport. In essence, AT ORP is a paradigm shift in corrosion stress management and control for the power industry.

AT ORP enables in-situ measurement of ORP in feedwater at the elevated temperatures where sensing electrodes are actually in contact with high-temperature water. Traditional ORP measurements utilize a nonrepresentative sample (cooled feedwater) that moves through long sample lines (increasing lag time) to a non-optimum device (RT ORP). AT ORP takes a representative sample (hot feedwater) to a much more sensitive device (Figure 1).

1.    At-temperature oxidation-reduction potential (AT ORP) measurement. These are the components of Nalco’s AT ORP cell. Courtesy: Nalco

The Nalco Co.’s AT ORP assembly consists of a sample inlet and outlet, connections for an integral resistance temperature detector (RTD), platinum electrode, and a separate external pressure-balanced reference electrode (EPBRE). This apparatus has been used in high-temperature water environments up to 500F and 3,000 psi (20.68 MPa).

The potentials measured by the AT ORP platinum probe are measured against the external, pressure-balanced, silver/silver chloride reference electrode. The temperature of the water flowing through the probe is recorded. All of the data collected by the probe can be archived and recovered on the plant data-logging system with suitable electrical signal conditioning. The signals can be used to control any species affecting the ORP space (typically reductant feed) and for advanced diagnostics.