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Why is corrosion important?

According to the Association of Materials Protection and Performance (AMPP) the total estimated annual cost of corrosion is as high as 3.5% of a country’s GDP [1]. An AMPP international study [2] found that in the United States alone, the corrosion related cost can be as high as $1.4 billion USD annually in the oil and gas exploration and production sector. This figure climbs even higher, up to $40 billion USD for gas and drinking water distribution plus sewer systems. This is an unavoidable problem with a high cost to bear.

Even though the corrosion itself isn’t unavoidable, it can be controlled by using the right material in the right place. Using a reliable test method that evaluates the material’s resistance against corrosion and predicts its potential failure is of the utmost importance. This test method should also be cost-effective and practicable.

What is corrosion?

Corrosion refers to a naturally occurring process that involves the deterioration or degradation of metals and alloys through a chemical reaction. The corrosion rate is highly dependent on the type of material, ambient temperature, contaminants/impurities, and other environmental factors. Most corrosion phenomena are electrochemical in nature and consist of at least two reactions on the surface of the metals or alloys.

For example:

These electrochemical process require three main elements:

  • Anode: where the metal corrosion occurs.
  • Cathode: the electrical conductor, which is not consumed during the corrosion process in the real-life electrochemical cell configuration.
  • Electrolyte: the corrosive medium that enables the transfer of electrons between the anode and the cathode.

Depending on the materials and environment, corrosion can occur in different ways, such as uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, or microbiologically induced corrosion to name just a few. Learn more about the different types of corrosion in our free white paper.

This white paper also includes details about relevant electrochemical techniques including Linear Sweep Voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Noise (ECN or ZRA). These techniques allow for the exploration of corrosion mechanisms, the behavior of different materials, the rate at which corrosion occurs, and also to determine the suitability of the corrosion protection solutions such as protective coatings and inhibitors, among others.

Find out more about these subjects individually with our selection of free Application Notes (AN).
Calculation of corrosion parameters with NOVA – Tafel plot corresponding to corrosion behavior of iron in seawater. (Click to enlarge)

Creating pipe-flow conditions in your corrosion laboratory

Internal corrosion is the most problematic cause of pipeline failure. To understand the fundamentals about corrosion failure and its root causes within pipelines, a similar environment should be created in the lab.

The Rotating Cylinder Electrode (RCE) is an integral part of creating hydrodynamic electrochemical experiments in the lab that create turbulent flow conditions which realistically simulate the situation for liquids flowing through pipes. The RCE can be used with most electrochemical techniques such as chronoamperometry, chronopotentiometry, and potential sweep.

Study of the corrosion rate as a function of rotation speed (convective flux) is one of the most common applications for the RCE. Corrosion studies can be performed using linear or cyclic polarization measurements (LP, DPD, CP), electrochemical impedance spectroscopy (EIS), and electrochemical noise (ECN) with respect to the rotation speed.

Results obtained by electrochemical methods are more accurate and are obtained much faster than conventional corrosion investigation methods (e.g. salt spray), providing more efficiency and productivity to any corrosion measurement laboratory. Learn about the RCE and how to simulate realistic pipe-flow conditions in the lab combined with electrochemical corrosion techniques in our free white paper.

One typical method in electrochemical corrosion studies is linear polarization (LP). With this method, it is possible to evaluate the corrosion behavior of a sample under pipe-flow (i.e. turbulent flow) conditions and learn about the corrosion rate of the sample at a specific flow rate.

Metrohm offers two Application Notes that use this technique specifically:

The Tafel plot obtained from LP measurement gives an indication of the corrosion potential. Using dedicated analysis tools in the NOVA software from Metrohm Autolab, the corrosion rate analysis can be performed and corrosion rate can be calculated, giving an indication of how much the pipe will rust in a year (in mm/year) under given conditions. Once this information is available for a certain material, a more corrosion resistive environment can be developed by applying a certain coating or a corrosion inhibitor.

Tafel plot created by Metrohm Autolab’s NOVA software. Blue line is measured without corrosion inhibitor and red line is measured with corrosion inhibitor.
Tafel plot created by Metrohm Autolab’s NOVA software corresponding to the measurements done in quiescent electrolyte (blue) and under 500 RPM rotation rate (red). All other experimental parameters were kept the same.

A second evaluation can be performed to learn how much the pipe will rust in a year, under these resistive conditions. In the example below, under standard conditions, the corrosion rate of carbon steel is measured at 0.25 mm/yr. However, when a specific corrosion inhibitor is used (tryptamine in this case), the performance is significantly improved and the corrosion rate drops to 0.065 mm/yr. These results can be achieved in a matter of minutes by using electrochemical methods, whereas by conventional methods (e.g., salt spray chamber combined with weight loss analysis), it takes up to a few months to conclude the results. That is a huge difference in efficiency!

Corrosion Parameter No Inhibitor With Inhibitor
Ecorr (V) from linear regression -0.479 -0.392
Ecorr (V) from Tafel analysis -0.482 -0.396
Rp (Ω) from linear regression 42.62 135.96
Rp (Ω) from Tafel analysis 43.32 136.39
Corrosion rate (mm/year) from Tafel analysis 0.25 0.065
Linear regression and Tafel analysis data resulting from experiments with and without corrosion inhibitor.

Summary

Understanding the corrosion behavior of a material under real-life conditions helps manufacturers to more quickly optimize the material design in terms of corrosion resistance, either by using a more suitable material for the pipes or by using adequate corrosion protection methods (i.e., coatings or corrosion inhibitors), which results in significant cost savings and safer operation.

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.

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