Corrosion Monitoring and Inspection – Is There a Difference?

Introduction

To many people involved in the Oil and Gas production and refining industry, the terms monitoring and inspection are used interchangeably when referring to corrosion issues. However, this lack of differentiation can lead to misunderstandings and errors.  It is our contention that a clear differentiation is needed and that engineers should strive to use the correct terminology.  In order to achieve that differentiation it is necessary to first define these terms ‘corrosion monitoring’ and ‘inspection’.  A review of some of the named techniques and methods used in these areas will help to consolidate an understanding of which terms fall into the inspection bracket and which are viewed as corrosion monitoring devices.

Definitions

The following definitions may not be exactly scientific in nature, but they do help to show two major differences between the two sets of valuable corrosion management tools.

‘Corrosion monitoring’ – is a way of determining how corrosive the fluids are within a specific environment. The various techniques available are typically used to give frequent, short time interval measurements, thereby allowing the day-to-day control of corrosion mitigation / prevention approaches such as corrosion inhibition. (The one exception to the ‘short time interval’ description is the weight-loss coupon).

‘Inspection’ – is the means by which corrosion (and other) damage may be located in a structure, as well as gaining insight to the amount and severity of that damage. Usually inspection tools are used less frequently than corrosion monitoring devices, often on an annual or even longer basis. However, the frequency of measurement should be determined via a process of risk based analysis to give a programme of ‘risk based inspection’ (RBI).

The Methods and Techniques

It is important to point out at the outset that the use of any corrosion monitoring device or inspection tool should be within the bounds of prudent process safety engineering. In the first place only trained personnel should be allowed to operate and maintain the various pieces of equipment. Secondly, they should learn about the system to be monitored / inspected, so that they clearly understand what risks are involved with respect to carrying out their monitoring / inspection activities.

Corrosion Monitoring Techniques

The most commonly used corrosion monitoring devices are included in the following list of equipment:
– Weight-loss coupons
– Spool pieces
– Electrical resistance probes
– Linear polarisation probes
– Galvanic probes
– Hydrogen pressure probes
– Hydrogen electrochemical patch probes
– Electrochemical noise probes
– Field Signature MethodTM
– Bioprobes

The majority of these techniques are classed as ‘intrusive’, in that for internal measurement there must be an access fitting to allow the measuring probe to be inserted into the process fluids. The exceptions on this list are the hydrogen electrochemical patch probes and the Field Signature MethodTM, which are attached to the outside surface of vessels and pipes.

Weight loss coupons, spool pieces and bioprobes, give one-off readings. In order to determine the result, each of these items must be withdrawn from the system and carefully examined and tested. The other devices can be left in place for some time, measurements being obtained either manually or automatically collected via hard wire connections or radio transmission devices.

Inspection Tools

The inspection tools are either arranged on the external surfaces of structures, or inserted into tubing via wireline and into pipelines installed in ‘intelligent pigs’.  The following list of these methods is not exclusive:
– Visual inspection
– Eye and magnifying glass
– Boroscopes
– Fiberscopes
– Robotic crawlers
– Cameras
– Calliper tools (on wireline or in intelligent pigs)
– Ultrasonic thickness (UT) measurements
– ‘Spot’ UT (compression mode) / straight beam (UTL)
– Pulse-echo contact method
– Shearwave mode (UTS)
– Phased array
– Automated UT (both in compression and shear modes)
– Long range UT (LRUT, or guided wave inspection – GWI)
– Radiography (RT)
– Dye penetrant (PT)
– Magnetic flux leakage (for example in intelligent pigs)

These first six methods are the most commonly used.  Others include the following:
– Dry magnetic particle
– Wet magnetic particle
– Wet fluorescent magnetic particle testing (WFMP or WFMT)
– Magnetostrictive guided wave testing (MGWT)
– Eddy current
– Pulsed eddy current (PEC)
– Neutron backscatter (for CUI – corrosion under insulation)
– Tangential radioscopy
– Magnetic flux exclusion
– Acoustic emission (AE)
– Acousto ultrasonics

Specialist application of some of these inspection tools allows the detection of cracking damage, including sub-surface cracks.  Early detection of the latter can obviously prevent subsequent catastrophic failures.

Finally

This ‘Tip of the Month’ has concentrated on the need to differentiate between corrosion monitoring and inspection, to show that each play a part in the overall corrosion management of an oil and gas production / processing system. For more information on these various methods there are a number of sources, including the JMC/PetroSkills training class:
“PF-22; Corrosion Management in Production / Processing Operations”.

By: Alan Foster
Discipline Manager for ‘Water and Corrosion’