What is Fretting Corrosion?
Galvanized sheet surfaces sometimes exhibit a surface imperfection that appears as permanent black spots, marks, lines, or patches. This defect has many names, including transit abrasion, friction oxidation, wear oxidation, and chafing; all being terms for a form of erosion-corrosion known as fretting. It is a phenomenon that is more commonly seen on metal surfaces in mechanical assemblies (e.g., bolted, riveted, keyed, or pinned joints, bearings) and electrical contacts, but can occur on galvanized.
While superficial, black fretting marks on galvanized sheet are almost impossible to remove, and are not the direct result of bulk water damage – which can also cause black (along with white) stains in its most severe form. When fretting occurs on galvanized sheet surfaces, liquid water is not necessary for its creation, although fretting can occur in the same areas of sheets that are additionally damaged by storage stain from entrapped moisture.
The requirements for fretting corrosion are: the interface must be under load, vibration or repeated relative motion must occur, and the load and relative motion must be sufficient to produce deformation on the surface. Displacements as little as 4x10-9 in [10-4 µm] can cause fretting. It is seldom seen above amplitudes of 0.001 in [25 µm] and reaches a maximum at 0.0003 in [7.5 µm].
The reason fretting damage can be a severe problem is that it so often happens at the interface of two highly loaded surfaces that are not designed to move against each other. For many years fretting problems have been observed on galvanized steel in both coil form and bundles of cut sheets. The defect is never seen at the production line and when found is almost always at customer facilities. It tends to be more prevalent on coil form and on material thicker than 0.030 in [0.8 mm].
Without fail, it is also characterized by a lower intensity mirror image on the reverse side of the sheet.
How to Minimize Fretting Corrosion?
Minimizing Fretting Corrosion on Galvanized Sheet There are a host of preventive measures that can be taken to minimize fretting corrosion. These include: lubricating with low viscosity oils or greases, optimizing the surface roughness to alter friction coefficients, isolating from vibration, increasing the recoiling tension or load to reduce slip, and decreasing the load at bearing surfaces.
All of the above measures are not practical in the case of galvanized sheet, but investigations at one steel supplier have indicated that some are effective to varying degrees. An action that has met with some success is redesigning support saddles to reduce concentrated point loading on the bottom of coils. By distributing the weight of the coil over the entire area of the saddle(s), there is less pressure at any one point, resulting in less transit damage given that vibration will always be present.
A slightly less effective way of accomplishing the same result is to reduce the coil size, but this is perhaps not a desirable option for all situations.
With either of these actions, care should be taken to avoid stacking coils during transit, as material on the bottom could become overloaded, even with well-designed saddles under them. If large, heavy coils are required to be shipped as-is, then a third option is to ship in the “eye-to-the-sky” orientation. This way, relative motion between the surfaces is eliminated, thus fretting is not possible. However, this method requires specialized handling equipment at both ends of the shipping route to rotate the coils.
Still another option to reduce fretting is to oil the sheet, thereby reducing friction. However, oiling has been found not to be effective in all circumstances and has other drawbacks, such as telescoping of coil walls, oil oozing from the walls, and being unacceptable to the customer.
An obvious cure would be to eliminate the small amplitude vibration of horizontally oriented coiled or stacked galvanize sheet. Accomplishing this is very unlikely given the nature of long distance shipping methods.
Sheet with Fretting Marks –
Suitability for Use Fretting marks on galvanized sheet are a surface oxide phenomenon that can be a major aesthetic issue, but there is no evidence they have a negative affect on corrosion resistance. Bright galvanize has a covering layer of zinc oxide that is not visible, whereas any fretted spots have an oxide layer that is black. This being the case, the product can generally be used in situations where appearance is not a factor, e.g., hidden structural members. In fact, specification EN 10326 Continuously hot-dip coated strip and sheet of structural steels, technical delivery conditions; states in clause 11.2 that darks spots resulting from friction during shipping generally only impair the appearance.
Unsightly white/grey/black storage stains and black marks due to fretting corrosion can result in rejection of galvanized sheet and products made from it. With proper attention paid to shipping and storage practices, these rejections can be minimized or even eliminated.