Research engineers at Bethlehem developed Galvalume sheet in the mid-1960's.  Introduced commercially in 1972, it has enjoyed the fatest gtrowth rate of any steel product on the world market.

The long-lasting resistance of bare Galvalume sheet steel to atmoshperic corrosion is well established throughout the metal building industry. Results are conclusive:  Bare Galvalume sheet is at least two to four times more resistant to atmospheric corosion than G90 galvanized sheet--and at least comparable to aluminum-coated Type 2 sheet durability-while providing superior galvanic protection and crevice corrosion resistance.

It is not surprising, then that prepainted Galvalume sheet is also demonstrating excellent resistance to corrosion.  In laborartory tests, as well as in controlled, real-life tests throughout the United States, prepainted Galvalume sheet outperforms prepainted galvanized G90 sheet.

Flexibility and paint adherence

the accompanying photographs below are 1.3 X magnification views of prepainted sheet deformed by an 80-inch-pound reverse impact.  A tape-pull has been used to evaluate the paint adherence after deformation.  Prepainted extra-smooth Galvalume sheet is on the left and prepainted minimized -spangle, extra-smooth galvanized sheet is on the right.  Both substances were painted with the same primer systems and topcoats.  The Galvalume sheet was painted with a chromate pretreatment, while the galvanized sheet was painted with a complex oxide pretreatment.

Prepainted Galvalume sheet is seen to be less susceptible to micro-crazing and loss of paint adhesion during forming.  This finding is consistent with laboratory forming evaluations which have demonstrated that the 55% aluminuim-zinc coating can withstand higher strain levels before a crack penetrates to the base steel, compared to a 100% zinc coating.  Thus, prepainted Galvalume sheet can more easily accomodate the strains encountered in typical roll forming operations used to manufacture metal panel components.  Such paint formability benefits are important for protection against atmospheric corrosion.

After eight years' exposure at four different test sites, prepainted Galvalume sheet exhibits superior overall performance compared to prepainted galvanized G90 sheet.  Each material was painted with Bethlehem-approved paint systems.  The only difference in processing was the use of a chromate pretreatment for Galvalume sheet versus a complex oxide pretreatment for the galvanized sheet.

Galvalume sheet vs. galvanized sheet

At the relatively severe Kure Beach, NC exposure site, 250 meters from the surf of the Atlantic Ocean, prepainted Galvalume sheet, left, exhibits excellent corrosion protection along a scribe used to simulate panel damage.  By contrast, the galvanized G90 sheet, right, exhibits light rust staining and the onset of paint creepback from the scribe.  Also of concern, is the appearance of microblisters over the enytire surface of the sheet.  (Photographs below are at 4 X magnification.)

Prepainted Galvalume Sheet     Prepainted galvanized G90 sheet

Edge Creep

Edge Creep can be fefined as a paint undercutting along sheared edges on prepainted metal sheeps.  Bethlehem's prepainted Galvalume sheet exhibits consistently minimal edge creep which tends to taper off to less than a millimeter after eight years' exposure at our most corrosive test site.  Adherence to Bethlem's Process specification is reflected in the excellent, long-term behavior.

By contrast, galvanized G90 sheet, prepainted with conventional paint systems, exhibits more variable edge creep performance.  the overall range of edge creep corrosion has also begun to increase beyond 6 years' exposure.