General or Uniform Attack

Designing in a system a corrosion allowance based on the possible loss of a material thickness is one of the simplest methods for dealing with uniform attack. Ultrasonic inspection has been used for decades to measure the thickness of solid objects. A piezoelectric crystal serves as a transducer to oscillate at high frequencies, coupled directly or indirectly to one surface of the object whose thickness is to be measured. The time the wave of known velocity takes to travel through the material is used to determine its thickness. Since the late 1970s, 

Rugged instruments based on portable computers are now available from many vendors. These systems, complete with motor-driven robotic devices to manipulate the transducer(s), have created the ability to measure wall thickness of corroded components at tens of thousands of points over 0.1 m, which can be converted into mass loss and pitting rates. This capabihty, coupled with increased precision of field measurements achievable with computer-controlled systems, has made these automated ultrasonic systems well suited for online corrosion monitoring [4]. 

Developments are now being made with individual transducers or transducer arrays that are left in place to provide continuous monitoring. Permanently attached transducers improve accuracy by removing errors in relocating a transducer to exactly the same point with exactly the same couplant thickness. With proper transducer selection, equipment setup, and controlled temperature conditions, the accuracy of controlled ultrasonic inspection can exceed 0.025 mm in a laboratory setting. Field inspections are typically to within 0.1 mm. 

The thickness of the metal substrate is determined simply by the time of flight for the ultrasonic signal to reach the back surface and 

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All forms of corrosion must be considered at the beginning of any test program, before discounting attack modes that are not likely to occur. Corrosion types can be divided into general (or uniform) attack and localized corrosion, in which extensive attack can occur in a very small area. Localized corrosion can be more difficult to observe. 

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