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Measurement of Effects of Erosion and Corrosion

DeFelsko Corporation

DeFelsko manufactures a hand-held, non-destructive ultrasonic thickness gage that is ideal for measuring the effects of erosion and corrosion on various substrates.

When evaluating the effects of erosion and corrosion on a wide variety of substrates, the primary objective is to non-destructively measure the remaining thickness of the substrate. Dependent on the application (i.e. size of the area to measured), rapid sampling and data logging may be required. Secondary challenges include attaining accurate measurements on substrates with rough or curved surfaces such as tanks and pipes.

In addition to substrate measurement, a means to determine the thickness of coatings applied to protect the substrate from handling and environmental elements may be required.

Substrate Thickness Measurement Solutions
The ultrasonic PosiTector UTG is ideal for non-destructive measurement of steel substrate thickness between 1 and 125 mm (0.040 and 5.00"). The lower picture (right) is an application of the instrument measuring the substrate thickness of the leading edge of a refurbished propeller blade. By measuring across the critical areas of the blade it is possible to identify areas that have experienced significant erosion or corrosion.

Image of Measurement of Effects of Erosion,Corrosion


The PosiTector UTG features several preprogrammed sound velocities for common substrates such as 1018 steel, stainless steel, cast iron, aluminum and plastics.  The user may either select a specific substrate from the gage menu or make a simple adjustment to a known thickness or sound velocity. To maximize the number of applications, the PosiTector UTG has a sound velocity range of 0.0492 to 0.393 mils/µs (1250 to 10000 m/s)

When multiple measurements are required, the instrument can be put into Scan Mode which enables 20 readings per second to be taken while the coupled probe is dragged across the desired measurement area. The Scan Mode of the instrument displays the total number of readings as well as the highest and lowest readings. Not only will this provide the user with a sampling of the thinnest and thickest readings attained from the substrate, but the readings can also be stored and downloaded to optional PosiSoft computer software by the instrument.

Prior to measuring on a rough surface it is recommended to remove any foreign debris such as rust or scale. A generous amount of couplant may be used to minimize surface effects while protecting the surface of the transducer from wear when dragging across the surface.

The PosiTector UTG will provide best results on surfaces with a radius greater than 50mm (2"). Measurements may be taken on surfaces of smaller radii by orientating the probe so that the dividing line on the surface of the probe transducer is perpendicular to the curve.

Dry Film Thickness Measurement Solutions
The PosiTector 6000 series of gages are ideal for non-destructive measurement of protective coating thickness on metal substrates.    For non ferrous substrates such as aluminum, the "N" series of gages correlate eddy currents from an induced magnetic field into a coating thickness. For ferrous substrates such as steel, the "F" series of gages correlate the change in magnetic flux density at the surface of a magnetic probe as it is brought near the substrate.  A complete range of gages and probes are available, each optimized for measuring a particular coating thickness on various coating/substrate combinations.

Application and causes of measurement
Erosion is the process by which a protective coating or substrate is worn away by friction resulting from repetitive mechanical interaction. Typical causes of erosion include cavitation, impingement by liquid or solid particles, and relative motion against contacting solid surfaces or fluids.

Corrosion is the process by which a substrate and its properties are damaged or worn away by a chemical action or change.  In metals, deterioration attributed to corrosion is most often caused by an oxidation process.  

With ultrasonic thickness gages, an accurate measurement of the remaining wall thickness of a substrate can be taken on pipes, pressure vessels, storage tanks, boilers or other equipment prone to erosion or corrosion. If within the 150 °C (300 °F) surface temperature constraints of the gage, most inspections can occur while the equipment is running, therefore avoiding loss of production.  

Though many industries are affected by erosion and corrosion, the Marine atmosphere is one of the most aggressive corrosion environments.  Corrosion rates are affected by several elements including sea water, humidity, wind, temperature, airborne contaminants, and biological organisms.  Erosion is also common in marine applications due to abrasion from the impact of water and contaminate particles, impingement due to turbulence in high speed liquids, and cavitation due to pressure waves produced by air bubbles. Erosion not only affects the substrate itself but may also damage protective coatings, increasing the likelihood of substrate corrosion.   Ships, marinas, pipelines, offshore structures, and desalination plants are all systems that are subject to varying levels of marine erosion and corrosion.

Ultrasonic Thickness Gages are used to measure a wide range of substrates and applications for loss of thickness due to corrosion or erosion.  Gages are designed for measuring thickness of metallic (cast iron, steel and aluminum) and non-metallic (ceramics, plastics and glass) substrates and any other good ultrasonic wave-conductor provided it has relatively parallel top and bottom surfaces.

An ultrasonic thickness gage facilitates rapid inspection of the thickness of large metallic structures at small measurement intervals, providing a high-detail thickness map of a scanned surface.  When access is only available from one side of the substrate, ultrasonic wall thickness measurement is the most efficient way to monitor effects of erosion or corrosion.

Companies using non-destructive inspection methods minimize safety concerns, ensure code compliance, and reduce the frequency of major repairs (and subsequently costs).  As an example, marine applications have a significant risk of catastrophic substrate failure due to undetected substrate corrosion or erosion.  However, costs associated with corrosion or erosion damage can be more subtle.  Consider the case of a propeller blade that has experienced wear or damage.  A likely impact is a decrease in the efficiency of the propeller, translating directly to a decrease in horsepower and an increase turbulence (vibration).   This results in a decrease in maximum speed and an increase in fuel consumption. Furthermore, cavitation caused by the damaged propeller creates a surrounding environment that is even more damaging to the propeller itself.

In addition to measuring effects of corrosion and erosion, ultrasonic thickness gages are often used in wall thickness measurement; detection of cracks, flaws and pitting; inspection of composite de-lamination; and the evaluation of welded or brazed joints.