Corrosion of metals is sometimes inactive, sometimes active. Some museum objects are susceptible to corrosion but remain stable because corrosion does not develop, while others are subject to active corrosion. Recognizing the early signs of active and destructive corrosion is an important aspect of preventive metal conservation. Few metal objects are immune to corrosion. Metals such as gold and platinum are the only ones that retain a bright, fully metallic appearance over a long period of time.
Active corrosion leads to a continuous loss of substance in the objects and requires measures to slow down or prevent the damage.
Inactive corrosion, on the other hand, appears as a stable oxide layer, which is distinguished by a gradual discoloration of metal objects and provides the underlying surfaces with some protection. The oxide layer is generally recognized as a desirable patina feature, especially when it has an attractive appearance. Artificial patinas are frequently applied to the surface of metal objects to change their aesthetics and to protect them. In addition, in museums, these various artificial patinas are mainly found on the surface of sculptures, weapons, tools and medals.
Corrosion detection is important in cultural heritage assessment. Many structures contain metal targets embedded in masonry or mortar, and corrosion can cause significant damage. However, detection by non-destructive methods is difficult and localized, providing in most cases incomplete results. In order to obtain a more extensive analysis, ground-penetrating radar was applied and evaluated to detect corrosion damage.
What is GPR?
Ground penetrating radar (GPR) is used in many areas to observe man-made and natural features. It allows for corrosion mapping, detection of metallic and non-metallic pipes, power lines, corrosion detection, detection of underground conduits such as water pipes, rebar and post-tensioning cables inside the concrete.
Ground-penetrating radar waves are equal to those of a cell phone or Wi-Fi network, while x-rays require a 50-foot clearance before being used for safety reasons. In general, ground-penetrating radar is the most cost-effective option and the fastest method of testing concrete.
The principle of using radio waves to determine internal ground structures has been known for a long time. Among the earliest works in this field, the use of radio echo sounders to determine the thickness of ice sheets in Antarctica and the Arctic and to measure the thickness of glaciers is undoubtedly the most successful. Ground-penetrating radar sensing in non-glacial areas was initiated in the early 1970s. The first achievements focused on work on permafrost soils.
Causes of corrosion mapping
Due to the localized nature of corrosion in sprinkler systems, corrosion rates vary considerably throughout the piping system. In sprinkler systems, the corrosion rate will be highest at the point adjacent to the air trapped in the system piping. In dry pipe sprinkler systems, the corrosion rate will be highest where the water is trapped.
This place in every system where air meets water is called the air/water interface. When oxygen from the air dissolves in water, it immediately reacts with the first steel it touches to form iron oxide. The air/water interface is the most active site of corrosion in these systems. At points in the piping system where the fluid is present and which are further away from the trapped air, corrosion rates will be quite low.
Corrosion Mapping – Invisible Corrosion
Hidden corrosion is a type of electrochemical degradation of materials that is not easily detected visually or by any other surface measurement technique. It can often be detected and quantified in terms of reduced wall thickness or structural discontinuities such as pits, defects and voids. When attempting to detect material degradation due to electrochemical processes, the corrosion products (e.g. iron oxides, aluminum oxides, etc.) must be identified in order to select an appropriate energy source for detection.
To inspect for invisible corrosion, the detection energy source must be able to penetrate the material in which the corrosion is hidden. If the appropriate source is selected, the returned signal will contain an assessment of the entire material, including the physical geometry of the component or system, which can indicate its structural integrity and any invisible corrosion. Thus, the inherent technical challenges are to select the most appropriate interrogation energy source and to recover the signal that identifies the existence of corrosion.
How to prevent corrosion?
Apply protective coatings: This creates a protective layer on industrial equipment, which greatly reduces the risk of exposure to corrosion factors.
Replace metal with other materials: To avoid corrosion, you can replace parts with other materials such as concrete, plastic, etc. In fact, this method is not always possible to apply because many structures are unable to do without the characteristics of metal.
Clean the working environment: By keeping your working environment clean, chemical reactions are less rapid and the risk of corrosion is lower.
Generating surface oxidation: By generating surface oxidation, metal corrosion can be prevented.