Metal Samples offers probes and inside corrosion monitoring including: metal loss coupons, coupons supports, access devices, tools to insert and remove coupons and probes, electric resistance probes (ER) and linear polarization resistance (LPR), biological probes and special technique probes, instrumentation to measure probes among others.
But what is corrosion monitoring?
Corrosion monitoring is the practice of measuring the corrosivity of a processed fluid or the degradation of the material through the use of corrosion probes or corrosimetric probes, and chemical and microbiological analysis. The probes or testers are inserted or connected to the equipment and are continually exposed to the process flow conditions.
There is a series of corrosion monitoring techniques which are used in corrosion engineering, among which, some may be followed online through constant process monitoring, while others can be determined through a laboratory analysis. Two of those monitoring techniques deserve highlighting: the one that uses metal loss corrosion coupons and electric resistance. None of the techniques are electrochemical and both are summarized below.
Corrosion Coupon (metal loss)
The metal loss corrosion coupons provide in a direct and simple way the corrosion rates of lines and equipment manufactured in carbon steel or other materials. Their methodology is relatively inexpensive and they provide reliable data for a certain period of exposure, in addition to information about corrosion morphology.
The basic procedure consists of using a test sample (a coupon) or alloy according to the equipment material and place it so that it will be exposed to the same electrolyte or medium of the equipment that the client desires to assess. After a predetermined time period, the coupon is removed from the process, cleaned so that all the corrosion product is removed and weighted. The metal loss is converted into corrosion average rate, according to the following formula (ANSI NBR 6210:2008) :
rcorr = rate of corrosion expressed in the desired unit (see Table 1)
k = constant that depends on the desired unit (see Table 1)
W = metal loss, expressed in grams (g) - mi-mf
A = test sample exposed area (cm2)
t = exposure time (hours)
d = metal or alloy density (g/cm3)
NOTE: The areas covered by the support and insulator should be excluded
Table 1 – Factor of conversion (k) for the expression of the corrosion rate
Expression Conversion Factor (k)
Thousandth of inch per year [mpy] 3.45 x106
Millimeter per year [mm/year] 8.76 x104
Micrometer per year [µm/year] 8.76 x107
Gram per square meter per hour (g/m2.h) 1.00 x104 x d
Milligram by square decimeter per day [mdd] 2.40 x104 x d
Microgram per square meter per second [µg/(m2.s)] 2.78 x106 x d
NOTICE: d = density of the metal or alloy
The complete procedure of preparation, cleaning and determination of the rate of corrosion is described in the ANSI NBR 6210 standard: 2008.
One of the main advantages of that approach in the determination of the rate of corrosion is that we will be able to identify the morphology of the corrosion, that is, whether it is uniform or local, and in addition we will get the real thickness loss in the exposure time period.
Another advantage is that this technique is applicable to virtually all environments, such as the gaseous, the liquid and the ones that involve flow of solid particles.
A disadvantage of this method is that these data are a history, and as such they provide information on past performance; therefore they cannot provide short term data or quick answers.
Monitoring Electric Resistance (ER)
Electric resistance probes can be considered automatic metal loss probes, providing continuous metal loss data. Continuous monitoring with electric resistance probes provides a trend followed by the corrosion rate against time, allowing clear identification of the periods of increase or reduction of the corrosivity of the medium associated with the variables of the process such as flow rate, temperature, pressure, concentration variation etc. In spite of that, they cannot show corrosivity changes in the fluids or corrosion rates in short periods of time.
The technique through which corrosion rate is measured is applied from the area variation, caused by the corrosion of the cross section of the sensor element (probe) exposed to the medium which causes the variation in the electrical resistance of this sensor. The stress of the exposed area causes the cross section of the sensor to shrink, and therefore the section loss causes the electric resistance of the metal to vary. This approach is based on the following equation:
R = Resistance (ohm)
= resistivity of the material (ohm.cm)
L = Length (cm)
A = Area of the cross section (cm2)
The variations of electric resistance are measured cumulatively along time, having as a base the initial measure of the sensor.
Actually, the equipment used converts the resistance into section loss of the sensor element automatically, and this measurement is based on uniform corrosion.
The electric resistance probe can be used in any kind of medium to determine the uniform corrosion rate, but its results are only meaningful for the average of mass loss of the sensor, and it is not possible to read the instant corrosion rate.
Data can be collected in the following ways:
Manual collection on site:
The collected data can be stored in a data collection station (logger) for subsequent retrieval of the information;
Transmitted online, via satellite, radio or any other way of transmission.
As one of the main advantages of the approach, we can mention that this technique is sensitive to the operating changes and flow regime, as long as they do not take place in a too short interval of time and, in addition to that, through the profile of the corrosion rate, we can modify the process variables to reduce corrosion.
Its main disadvantage is the fact that the method is not very sensitive to localized corrosion.
Monitoring Linear Polarization Resistance (LPR)
The Linear Polarization Resistance method is based on principles and electrochemical concepts. The electrodes at the tip of the LPR sensors (two or three electrodes) are polarized in short intervals of time and as an answer to this polarization we have he corrosion rate in real time for the electrolyte, or medium under analysis. The Measurement principle of this technique is valid, because the current density associated to small polarization effects is directly proportional to the corrosion rate of the electrode in the medium that is being observed.
The main advantage of that technique is that we have a corrosion technique in real time and its main limitation is that the electrochemical principles that are applied in this technique are for uniform corrosion. If pitting corrosion is still present, the values will be distorted and not real.