Service failures can be driven by minute levels of corrosion
Measuring the rate of corrosion that occurs in metallic parts is important to avoid in service failures. Some components are made with a sizeable proportion of sacrificial material to allow for the effects of corrosion, but many others do not. Understanding the rate of corrosion allows for a pre-emptive replacement or redressing program. Traditional methods, such as weight loss analysis, allow assessment to be made of the overall loss of material caused by excessive corrosion. However, smaller defects may be visible and may start to form corroded pin holes. A surface with many small defects over time, is as dangerous as a surface displaying large defects.
Optical metrology provides the solution
Optical 3D surface metrology systems provide a solution for this question. Bruker Alicona provide a range of instruments that can meet this requirement. The smallest instrument in the range is the Bruker Alicona InfiniteFocus SL. In use the standard flat coupon sample, as shown below, is placed directly on the XY stage which can move 50 x 50mm allowing large areas to be measured, larger systems will accept 200x200mm.
3D data sets are then obtained providing a visual 3D image of the surface with the defects clearly displayed. This allows the user to examine the surfaces. The definition of large and small pits can easily be seen in the data sets shown below in Figure 2. The left-hand image showing large corrosion pits and the right one displaying small corrosion pits. This is more graphically displayed when the data is displayed in pseudo colour related to height in figure 3.
The large and small pits can be seen but the pseudo colour image makes the difference and degree of surface damage more apparent. However, using the Bruker Alicona system with built-in analysis provides full surface data which includes number of defects, maximum depth, maximum projected area, maximum true area, maximum volume, and area of defects as a percentage. As corrosion develops over time, the Infinite Focus SL’s database function allows images to be saved and compared with earlier ones. The rate of change and so the progression of the defects can be assessed.
There is often a requirement to assess round coupons. Using the optional Real3D rotation system automates carrying out full 3D measurement of the length and circumference of the coupon providing the same measurement capabilities as with the flat coupon.
Optical metrology provides the solution
It can be clearly seen that Optical 3D metrology can offer a solution to the measurement of micro pits caused by corrosion. In a simple to use package and not requiring any metrology knowledge to operate, users can easily scan a surface and measure pit size and volume, giving a quantitative measure of corrosion damage.
