Intermodal Eye has partnered with Bob De Bos from Robogrind to bring you a guide on the different types of tank container corrosion. Robogrind is a supplier of semi-automatic robots used to maintain the stainless steel inner tank shell.
Due to the properties of stainless steel tanks and the different products transported in them, the appearance of corrosion may differ on each occasion. Although Stainless steel has a level of corrosion resistence, it is not completely immune.
Corrosion can be caused by by various reactions. The most common form is an electrochemical reaction that might occur when the steel gets in contact with a fluid like water, acid, oils and liquid chemicals. It can even be caused by contact with moisture in the air.
It is important to recognise the appearance of the corrosion type so you can identify the best way of removing it. In this Blog article, we list the most common types of corrosion found in tank containers as well as other surface imperfections.
Shallow Corrosion (and wide)
Shallow corrosion can be recognised as a deepened surface with irregular edges that are not too sharp. The bottom is irregular shaped. Be aware that it could also contain more deep forms of corrosion. The shape is always wider than deep but it could still penetrate quite far into the tank.
Pitting Corrosion (Pinhole)
Pitting can be recognised as a deepened surface with circular and sharp edges. The colour of the hole depends on the relation between the depth and width of the pit. Black holes will be deep and narrow so light is unable to reach the bottom to reflect. The bottom of this corrosion type cannot always be seen other than with a loupe (a small magnification device).
Pitting Corrosion Caved (Cavity Pitting)
Cavity pitting can be recognised as a dark opening in the surface with circular and sharp edges. It is often surrounded by other smaller holes. Only research with a loupe might clarify if the pit is caved. This is the most dangerous type of corrosion as the visible corrosion could be repaired but the deeper pitting may not be noticed.
Uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area or a large fraction of the total area. It can look the same as shallow pitting but it could contain deeper damage.
Tool Marks (and scratches)
A tool mark is damage caused by a tool. It’s not a form of corrosion but it could cause the tank to corrode more easily (due to the damaged oxide layer). The shape depends on the tool causing the damage but it could have very sharp upstanding edges on which you can even cut yourself. It can be circular or a scratch of various lengths. Once the sharp edge is polished it could easily be confused with corrosion.
A roll mark is a tool mark made during the production of the stainless steel or when the plate is shaped into cylindrical form. Older tanks are prone to roll marks because they are “hot rolled”. Hot steel is softer during the manufacturing process and is damaged more easily. Dirt or irregularity on the pressing equipment causes this damage as well as the shot blasting process. Roll marks have a nice form and no sharp edges. Around the edge you will recognise a smooth circle of extra pressed steel in a different grey scale. Once the edge is taken away by polishing, it could easily look like corrosion. It is important for that reason to make a note about roll marks on estimates for polishing.
Intergranular Delamination looks like the top layer of the steel has come off and is very sharp. It is found mostly within areas of approximately 20cm around the horizontal welding lines of tank sections and could therefore be caused by welding combined with curving the plate in its cylindrical form. Delamination itself, seen as a stripe, may therefore always start in a horizontal direction. Although often requested, polishing does not remove delamination. It only takes away the sharp edges and the delamination process may go on.