Corrosion Monitoring in Piping Systems

Corrosion monitoring is one of the important requirements for safe and efficient plant operations and may be defined as a system to measure the corrosion occurring in a piping system within a specified period of time to ensure that the overall integrity of the piping system is preserved.

The control of corrosion in plant piping, equipment and pipelines is of paramount importance. Understanding the condition of the piping or pipeline and how this evolves over time is therefore an essential element of piping/pipeline integrity management. Corrosion monitoring of plant piping helps the owner identify the corrosivity of the medium, the corrosion and pitting rate and the efficacy of the corrosion control program that has been implemented for the plant facility.

Corrosion Monitoring Techniques

The following are the corrosion monitoring techniques employed in the industry: 

• Direct Intrusive
• Direct Non-Intrusive
• Indirect online monitoring
• Indirect offline monitoring

Direct Intrusive Method

In this method, the corrosion monitoring element is in direct contact with the process medium for monitoring the corrosion. Direct Intrusive methods are achieved using the following types of corrosion monitoring equipment's:

• Corrosion Coupons
• Electric Resistance Probes
• Linear Polarization Resistance Probes
• Bio Probes

Corrosion Coupons

The use of corrosion coupons is one of the most reliable technique of internal corrosion monitoring. A corrosion coupon comprises of a specimen of metal that is inserted into the process medium for a specified period of time to assess the extent of corrosion. The coupon should of the same material as the piping to get a representative measure of corrosion. The difference in weight of the corrosion coupon prior to installation and after exposing it to the process medium for specified period of time is used to determine the corrosion rate. After removing the coupon, it is visually inspected for any mechanical damage, corrosion damage, corrosion morphology and any deposits. The coupon is normally sent to a laboratory for measurement of corrosion rate.

Electric Resistance Probes

Electric Resistance (ER) Probes do not require weight measurement of coupons. The metal loss or corrosion rate is determined by measuring the change in electrical resistance of a probe element. 

Linear Polarization Resistance Probes

Linear Polarization Resistance (LPR) Probes also do not require weight measurement of coupons. The method is based on measuring the current response to a small polarization (10 - 20 mV) to a steel electrode’s corrosion potential. The limitation of the method is that the electrochemical basis for the method requires that a conductive electrolyte (water) is present. Hence, LPR Probes are mostly used in water systems.

Bio Probes

Sulfate Reduction Bacteria (SRB) and Thiosulphate Reducing Bacterial (TRB) are the major contributors to microbiologically induced corrosion (MIC) in oil and gas producing and water injection pipelines. Bio Probes offer an effective mechanism to collect samples for microbiological analysis in piping and pipeline systems susceptible to microbiologically induced corrosion (MIC). 

Direct Non-Intrusive Method

In this method, the corrosion monitoring element is not in direct contact with the process medium. The corrosion impact is measured from the external surface of the pipe that contains the process fluid. Non-intrusive methods are specified for process fluids which are highly hazardous or lethal where retrieval of the corrosion monitoring element can create a potentially dangerous situation.

Ultrasonic Monitoring

In this method of monitoring, an array of piezoelectrically active transducers are attached to a flexible backing strip which is strapped or permanently bonded on the pipe or equipment. Corrosion mapping involves scanning with one or more straight beam ultrasonic probes over a pipe surface to capture thickness measurements. The measurements are converted into digital values which are color-coded to create topographic maps of the pipe wall thickness profile. Images from individual scan areas are assembled together to create composite images covering large areas of material. Ultrasonic monitoring is used to monitor corrosion on insulated lines by installing the probes under the insulation. It can be used to monitor other inaccessible areas of piping.

A few examples of this technique which are commercially available are provided in table below.

Acoustic Emission Monitoring

The acoustic emission (AE) corrosion monitoring method is another non-intrusive method that involves the use of piezoelectric sensors, which are used to measure the elastic waves generated by the distortions produced in the corrosion process. The advantage of the AE system is that it can be used to locate damaged areas with a relatively minimal number of sensors. Another benefit is that its equipment can be setup in a relatively short time compared to radiography and ultrasonic testing which require larger set up time as well as inspection of entire area. AE method can be used for detecting onset of cracking and to monitor the growth of crack over time. 

Inductosense Technology Monitoring

The Wireless And Non-Destructive (WAND) system from Inductosense provides an modular approach to internal corrosion and erosion monitoring. The WAND system comprises of battery free ultrasonic thickness sensors that can be permanently installed on the piping. The thickness data from the sensors can be collected using either a handheld probe or a remote data collection device.

The handheld probe powers up the sensor by non-contact inductive coupling when it is bought in close proximity to the sensor. The sensor then sends out an ultrasonic wave into the piping and detects the received signal. The signal is transmitted back wirelessly to the probe where the signal is analyzed. The handheld probe displays the wall thickness and the sensor RFID information.

Indirect Online Monitoring Methods

This technique involves monitoring of process parameters that can have an influence on the corrosion risk to the facilities. This method includes hydrogen probes and patches, acoustic solid particle detectors, on-line process analysis and online process monitoring.

Indirect Offline Monitoring Methods

Offline monitoring methods involve sampling analysis of process parameters that can have an influence on the corrosion risk to the facilities. Sampling analysis is useful for qualitatively monitoring the extent of corrosion. Fluid samples are collected from different locations at regular set periods of time and the chemical composition of the fluid is used to measure the corrosivity of the medium.

Location of Corrosion Monitoring Elements

The location of corrosion monitoring elements is of paramount importance as wrong selection of location can give erroneous results and corrosion forecasts. If the pipeline system is protected using chemical injection system, the corrosion monitoring devices should be installed downstream of the injection points to check the efficacy of the chemical injection system. In a pipeline system, the corrosion monitoring may done at both launching and receiving end or only the receiving end depending on the corrosion monitoring philosophy.

Corrosion monitoring tools should be installed at low spots and long horizontal sections where accumulation of corrosive fluids is likely to take place. For example water is likely to accumulate at low spots in an oil pipeline due to higher density. It is recommended to install the monitoring devices at 6 o'clock position to facilitate bottom of line corrosion monitoring. If this position is selected for monitoring, layout should ensure that sufficient space is available below the pipe to install a retrievable tool and service valve to retrieve the corrosion coupon without shutdown of the piping/pipeline system. There are some disadvantages associated with this installation position.

Permanent safe access should be provided for any location where corrosion probes need to be monitored, serviced, or replaced on-line.

The access fitting for corrosion monitoring should be installed such that straight length of 10D is available upstream and downstream of the coupon or probe. There should be no branch connections, reducers, valves in this straight length section.

The length of the coupon or probe should be selected based on the position within the pipeline that needs to be monitored. 

Manufacturer should carry out natural and wake frequency calculations to avoid fatigue failure of the monitoring connection due to resonance or drag force.

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