Galvanized sheet sometimes exhibits a surface imperfection that appears as short black marks, usually in patches.  This condition has several names in addition to transit abrasion including fretting corrosion, friction oxidation, wear oxidation and chafing.  These terms refer to the root cause of the problem, which is related to friction between contact points, similar to galling. The condition is characterized by a mirror image on the reverse side of the sheet. The reason the term corrosion or oxidation is used is that this imperfection is associated with the buildup of oxide particles.

Fretting corrosion is the most technical name.  It refers to corrosion damage at the high points of contact surfaces.  It occurs under load, under conditions of repeated relative motion of the surfaces in contact with each other, and these two conditions must be sufficient to produce deformation of the surface, which is likely with galvanized sheet because the zinc coating is fairly soft. This mechanism can affect any two surfaces that are not intended to move against each other and, in the case of machinery, can prematurely wear out parts.

Fretting corrosion has been observed on galvanized steel in both coil form and bundles of cut lengths.  This condition is never seen on the galvanizing line, almost always being found in a customer’s plant.  The repeated motion comes from vibrations that occur during shipment of the product.  The condition is rare in the case of truck shipments, generally being prevalent when product is transported by train and ship where it incurs vibrations for long periods of time. The load comes from the weight of the coil (or stack of sheets).  This is why transit abrasion is observed mostly on the outer portion of the coil at the bottom half of it (or bottom portion of the bundle).

There are measures that can prevent or minimize transit abrasion, all targeting reducing load or minimizing friction.  Actions that are very effective are designing support saddles to reduce concentrated point loading on the bottom of coils and avoiding stacking during transit.  Other measures are reducing the coil size and oiling the material, although these methods are not always practical or possible.

There are two mechanisms that operate to produce fretting corrosion:  wear-oxidation and oxidation-wear. The first proposes that cold welding occurs at the contact points with small fragments of metal being removed and that these immediately oxidize.  The second proposes that the normal oxide layer already present on the galvanized sheet is ruptured at the high points under load and vibration, thus producing oxide particles. 

Fretting corrosion is a cosmetic condition. There is no evidence that it is detrimental to the corrosion resistance of the galvanized sheet.

Pressure marking (or pressure mottling) is an uneven gloss pattern on the surface of a prepainted coil.  It is generally caused by dissimilarity in the gloss and in the roughness between the top and bottom surfaces of a coil.  These gloss differences are transferred with time and pressure to either surface.  Pressure marking is not a degradation of the paint surface except in extreme conditions.  The irregular pattern is typically temporary and will dissipate with time and exposure to heat or ambient temperature.

In the mechanism by which pressure marking occurs, the gloss components, which are under normal suspension within the topcoat and are random in directionality, are suppressed or flattened within the topcoat under the pressure of coiling. 

Coil coaters use various best practices related to the backer specification, rewind tension and coiling temperature in order to minimize pressure marking.

Storage stain on galvanized sheet steel is a corrosion product that is typically white, but which can also take the form of grey or black deposits on the surface. Since the most common form of discolouration is white in appearance, storage stain is often called “white rust.” It can occur when sheets of galvanized steel that are in contact with each other (in a coil or stacked in lifts/bundles) get wet, either by direct water impingement, or condensation between the surfaces, causing the zinc to react with moisture in the absence of free air circulation.

Passivation coatings have been in use for many years and are very effective in minimizing the tendency for corrosion when the sheets get wet in coil or bundle form. However, they do not eliminate its occurrence if the product is subject to very adverse conditions.  Steel sheet producers use the term “passivation treatment” or “chemical treatment” interchangeably for this surface application.

White rust is characterized by a mirror image appearance in between laps/sheets at the point at which the material became wet.

“Skin Lamination” associated with entrapped mold powder in sheet steel

“Skin lamination” is a term often used to describe surface imperfections of various types including that associated with entrapped mold powder.

Mold powder is an inherent part of the casting process; a consumable used to lubricate the mold and to protect the molten steel from the atmosphere.  Mold powder that is entrapped in the solidifying steel shell can be rolled out into a thin layer close beneath, or just at, the surface of the hot rolled sheet.  It sometimes becomes obvious only during forming.

Steelmakers endeavor, through careful control of the casting process, to keep the mold conditions as “quiet” as possible to minimize mold powder entrapment.  This includes maintaining precise control of the liquid steel level in the mold, and controlling steel flow in the mold. The process is monitored using an “automatic mold level control system” to eliminate the subjectivity associated with humans.  Detection of events by mold level measurement enables a decision-making process on acceptance for an order, which may lead to slab conditioning. 

• Coil Breaks are creases which appear as lines transverse to the rolling direction and generally extend across the width of the steel.
• If a coil is sheeted and used in flat form the condition is considered unsightly.
• Coil Breaks are purely cosmetic.They are non-injurious and will not affect formability.Coil Breaks will not affect the integrity of formed parts.The presence of Coil Breaks is a clear indication of very formable and ductile material.

What causes Coil Breaks?

• Coil Breaks are caused by the phenomenon of “yield point elongation” or “elongation at the yield point” which is inherent in low carbon steel.
• When a tension test specimen is subjected to load there is an initial range of loading in which no permanent deformation occurs, i.e. if the load is removed at any value within this range the specimen will return completely to its original dimensions (elastic range).
• As the tensile load on the specimen is increased through the elastic range a stress will be reached at which the specimen will begin to deform in a plastic manner, i.e. it will undergo a permanent set which is not recoverable upon release of the load.
• Some materials, with increasing stress, show a gradual departure from elastic behaviour.Many steels, however, exhibit an abrupt yielding and show an increase in strain without any appreciable increase of stress when yielding occurs.Such materials are said to have a yield point.The amount of extension which occurs between the initial yield point and the point at which the load begins to rise steadily again is called the “yield point elongation”.
• When hot rolled coils are uncoiled for further processing, such as pickling, slitting, cutting to length or temper rolling there are strains associated with the uncoiling.Localized yielding occurs at the point of unbending when the strain exceeds the yield point of the material.This is what Coil Breaks are.The strain is a function of the ratio of the thickness of the material to the diameter of the coil and it is greatest at the wraps closest to the inside diameter and so Coil Breaks are most prevalent there.
• Since the mechanism of formation of Coil Breaks is dependent on the yield point it is clear that materials with higher yield strength (e.g. HSLA) are far less susceptible to Coil Breaks.
• Other factors that affect coil breaks are:
  • Tension
  • Shape
  • Length of time allowed for cooling after hot rolling (longer is better)
• Coil Breaks will further occur at other points of bending, such as roller leveling.Roller leveler breaks are said to be controlled Coil Breaks in that they are caused by bending the sheet over regularly spaced rolls.

   

  Eliminating Coil Breaks

• Modification to chemistry with Boron to change the characteristics of yielding behaviour can minimize Coil Breaks.

• Skin rolling or temper rolling can be used to alleviate coil breaks in two ways.A small cold reduction is applied to create numerous nucleating points for Coil Breaks that are so small so as not to be obvious.In addition, pre-existing Coil Breaks are flattened and masked.
• In order to guarantee freedom from Coil Breaks on hot rolled steel the product must be skin rolled or temper rolled.
• Coil Breaks are not usually seen on fully processed cold rolled sheet because the processing includes temper rolling after annealing.Similarly coated products are either tension leveled or temper rolled in line.