High-tech tools help cut the costs of corrosion

Ian Verhappen
Industrial AutomationNetworks Inc.

Estimates from a variety of sources put the cost of corrosion at 3 to 5% of GDP, or more than $2 trillion per year. The economic impact of corrosion is more than "lost metal": it also includes lost production from unscheduled outages and potential risks to health, safety and the environment. And, as several pipeline companies have learned in the past few years, there is also the impact on one's social license to operate, which is then reflected in company share price. The offshore environment is more at risk to corrosion that land-based equipment due to salt water and the repetitive stress caused by wave motion.

Metallurgists are always striving to reduce this impact. However, it is often difficult to gather data to make informed decisions. Fortunately, new tools and technologies are becoming available to allow both real-time corrosion rate information as well as reconciliation of a range of data sources to provide useful information on the status of equipment at greater speeds than in the past.

A number of manufacturers have developed products to measure corrosion rates in real time, and in doing so are making the labor-intensive corrosion coupons that have been the backbone of industrial corrosion monitoring for more than 50 years obsolete. Corrosion coupons are simple to use and are usually accurate, but they are completely manual: the measurements must be made offline, and are labor intensive. Coupons must be pre-weighed, distributed to remote locations, installed, retrieved, examined, cleaned, and re-weighed before data is processed.

The new products take advantage of today's micro-processing power to use the information from single transmitters making multiple parallel measurements to a single output correlated to the corrosion rate in the vessel, pipe, or ducting as appropriate. The data is transmitted to a central computer, where it is integrated with other software to measure and monitor the corrosion rate and the effectiveness of countermeasures taken to minimize it. There are two forms of measurements:

• Electrical Resistance (ER) probes provide on-line data about corrosion rates to check the efficacy of corrosion inhibition or detect changes in underlying process corrosivity.
• Linear Polarization Resistance (LPR) is a fast-responding intrusive method for measuring changes in the corrosivity of aqueous solutions, though the presence of oil in the process stream may affect this form of measurement.

The measurements themselves quantify changes in current density due to material loss. Much like a pH meter, or Wheatstone bridge, a comparison is made between a sample and reference measurement to provide a signal-conditioned, calibrated output.

With real-time data available, it is possible correlate a spike in corrosion rate with a facility operating condition, ambient environment situation (rain, high wind, freezing, or high temperature) or time of day (sunrise, sunset, shift change, etc.). Appropriate actions can then be taken to reduce the source of the increased corrosion rate, prevent its recurrence, and thus reduce the overall annual or compound corrosion rate and extend the life of that component.

Because these sensors tend to be distributed in a wide range of locations, and in many cases where a signal cable may not be available, many of these monitoring systems rely on wireless technology to transmit the process reading. These wireless networks typically require installation of a backhaul of backbone to bring data from all the devices back to a central monitoring station, which in some cases is a distributed control system, or DCS. This same wireless network infrastructure, which is often based on IEEE 802.11 protocols (WiFi), can also be used to collect data from inspectors as they are doing their manual readings, such as ultrasonic testing at critical points for localized pitting, or cracking corrosion directly from their measurement tools to the corrosion monitoring system. Such measures help prevent transcription errors and reduce the time associated with manual data entry, so that inspectors may spend their time analyzing the data and planning corrective actions.

Online corrosion tools, by their nature, present an average corrosion rate, where average applies to the surface area of the "corrosion coupon" from which the measurements are made. As a result, other offline tools such as "smart pigs" and the abovementioned ultrasonic measurement tools are required to complement the measurements. Both of these techniques require a high degree of manual intervention either to launch and receive the pigs, or to place the ultrasonic test tool at the right locations on the pipe or vessel. Fortunately, both techniques are able to detect concentrated or localized corrosion rates that result in pinholes and associated leak sources.

The above tools are predominantly used to monitor for internal corrosion; unfortunately, the majority of metal in the oil and gas environment is either covered with insulation or submerged, so external corrosion cannot be observed simply by walking past and looking for rust spots. One way to estimate the corrosion rate under coatings is to correlate cathodic protection current with rate as an indicator of how much metal is being sacrificed to prevent potential corrosion. Once again, wireless technologies are often used to gather this information and send it back to a central repository for analysis.

More than just a little rust, corrosion has a big impact on reliable equipment operations, especially in the harsh offshore environment. We are fortunate that a number of related technologies are evolving to allow us to better track the impact of corrosion in real time, combined with better analytical and diagnostic off-line tools to reverse the corrosion trend, especially as some of our aging infrastructure is approaching its end of design life corrosion margins.

Author

Ian Verhappen, P.Eng. is an ISA Fellow, ISA Certified Automation Professional (CAP), and a recognized authority on process analyzer sample systems, Foundation Fieldbus and industrial communications technologies. Verhappen operates a global consultancy, Industrial Automation Networks Inc., specializing in field level industrial communications, process analytics and hydrocarbon facility automation. Feedback is always welcome via e-mail at[email protected].