9 Facts You Should Know About Fretting
Transit abrasion, friction or wear oxidation and chafing are all defects more commonly known as fretting.
Typically found on mechanical metal pieces like bolts, pinned joints and rivets, fretting can also occur on galvanized sheet during shipment.
These black spots, marks, lines and patches are certainly unsightly, but have you ever wondered if they affect the structural integrity of the steel underneath?
To find out, we pulled together nine facts about fretting on galvanized steel sheet.
Here’s what you need to know:
1. A mirrored image on the sheet points to fretting.
Many people think that fretting is a defect due to missing zinc or bare spots. Not so.
“Without fail,” says GalvInfo, “[fretting] is characterized by a lower intensity mirror image on the reverse side of the sheet.
If fretting were caused by processing, it would be sporadic, not stack on itself or mirrored.
2. Fretting is caused by vibrations during transit.
Fretting is never spotted in production, but instead only upon customer receipt, meaning that the defect occurs during transport.
But it’s not vibrations alone that cause fretting. GalvInfo notes that for fretting damage to take place, the material must be “under load and in the presence of relative surface motion, as often induced by vibration.”
Over time, this repeated friction can cause contact points to rupture, producing oxide debris.
3. Fret marks are black due to small zinc oxide particles.
Zinc oxide that’s formed from corrosion of zinc by oxygen in the atmosphere is a white powder.
So why are fret marks black?
Because a higher percentage of oxygen is found in fret marks than in normal white zinc, it’s thought that either this is a different form of zinc oxide, or very fine particles have bound to the surface of the underlying zinc.
4. Fret marks can not be removed.
Short of abrading the marks off, the black marks left behind by fretting are impossible to remove.
5. Fretting is more likely to occur on material thicker than 0.030, or 0.8mm.
The thicker the sheet, the easier it will slide against itself than the same weight of thinner sheets.
Because there is less total frictional force (per unit volume) to resist sliding, says GalvInfo.
In the case of coils, thinner sheet is recoiled tighter than thick sheet, giving it less chance to slide against each other during shipment.
6. Fretting is more common on material shipped by train or barge.
Because truck transport tends to involve shorter distances – and thus, fewer vibrational cycles – it’s more common to see fretting damage on steel shipped by barge or train.
The thought is that there’s more time to do damage on longer haul trips, and the movement over rails and through water may generate low amplitude vibrations that encourage fretting.
7. Fretting generally occurs in a specific region of the sheet.
Along with vibrational movements, load is also a large contributor to fretting.
Typically bearing points – such as the area of contact with cradling supports – show the most fret damage, indicating that the weight of the bundle or coil plays a role.
8. Fretting can be avoided.
In the case of galvanized sheet, there are a few ways to minimize transit abrasion damage, though none are 100% foolproof.
Firstly, redesigning support saddles to more evenly distribute the weight over the entire area of the saddle can be an effective way to prevent fretting.
Secondly, reducing coil sizes, or taking care to avoid stacking coils during transit can help to minimize transit abrasion.
Thirdly, oiling the sheet to reduce friction. However, GalvInfo reports that oiling has been found not to be effective in all circumstances and could present other drawbacks.
9. Fretting only impairs the appearance of galvanized sheet.
While fretting on mechanical parts can seriously hamper their ability to perform, fretting on the surface of galvanized sheet has no negative affect on corrosion resistance.
In circumstances where appearance is not a factor, GalvInfo notes that galvanized steel sheets and coils affected by fretting are still fine to use.