Dear Dr. Ingmar Bösing ,

localised corrosion can visually investigated by using specimens photographs and macro photographs taken with a high resolution digital camera after different exposure times, in a suitable and specific environment.

Evaluation of the number and area of the points of corrosion can also executed with ImageJ software. We selected and cut a surface of about 25 cm2 from the specimens photographs. After using the programme to process images for brightness and contrast, these areas can be examined through the “Analyze particles” software command.

Pits size can be deeply investigated using an optical metallographic microscope, equipped with a motorized specimenholder table with a display that can showed the movement along the Z-axis. In order to evaluate the maximum pit size we can considered an equivalent pits diameter and the pits depth.

The equivalent pits diameter can be calculated considering the pits boundary as perfect circumferences and using an eyepiece with a graduated reticle. The pits depth was evaluated considering the difference between the focal planes relative to the top and the bottom of each analysed pit.

The measure of the pit depth on the microscope display is visualised in whole increments of micrometre. Therefore, it is possible to make measurements along the Z-axis with a very high precision.

For studying corrosion morphology and chemical composition of corrosion products, the surfaces of the specimens have to first cleaned by blowing air and then analysed under a Scanning Electron Microscope with Electron Probe Micro.

After removing corrosion products by Keller's solution, the surfaces of the specimens can be also studied using a No Contact 3D Optical Profiler, collecting images that are processed with analysis software in order to evaluate pit dimension and depth.

My best regards, Pierluigi Traverso.

A common practice is to perform electrochemical tests in real solutions relevant to practice and see where the critical potentials are. Then one estimates (or measures) the redox potentials in the application or the maximum possible potentials of the material under practical conditions. There should be as large a distance as possible between the free corrosion potentials and the critical potentials (usable passive range). However, this only protects against initiation of localized corrosion. However, it is better (more conservative) to use the repassivation potential of the material under practical conditions, since initiations and disturbances of the passive layer can always occur in practice. Here, too, the free corrosion potentials in the application should be lower than the repassivation potentials.