Zinc Roofing Screws Susceptible To Corrosion

The majority of the fasteners used in buildings and roofing are carbon steel due to its hardness and strength.

Zinc roofing screws susceptible to corrosion.

When zinc contacts copper in the presence of an electrolyte such as water a galvanic reaction will lead to corrosion of the zinc and subsequent failure of the roof or wall cladding. Carbon steel however will quickly corrode when exposed to the elements unless coated with at least 7mils of zinc coating. You will note the roofing screws to the right where the galvanised sheet is on top of another galvanised sheet are not showing signs of corrosion. In the event of roof renewals where lead already exists and its re use is desirable the zincalume steel or colorbond steel must be insulated from the lead by a suitable barrier.

The problem is that this strength might only be skin deep so to speak. Select stainless steel fasteners or fasteners with long life coatings when practical. Now that s fine provided the zinc coating is never worn away somehow. This also the material of choice for self drillers.

How exactly does zinc protect screws from corrosion. Zinc is more corrosion resistant than steel in most natural atmospheres the exceptions being ventilated indoor atmospheres where the corrosion of both steel and zinc is extremely low and certain highly corrosive industrial atmospheres. Over time however this coating can be worn away even if only in one small spot leaving the rough steel underneath open to the elements. Well zinc can still corrode but it corrodes at a significantly slower rate than other metals and alloys.

Coatings of metallic zinc are generally regarded as the most economical means of protecting against corrosion. Pure zinc or aluminum are suitable alternatives although neither are as workable as lead. Run off from a copper surface to a zinc surface must be avoided under all circumstances. Unfortunately all of these metals and alloys are vulnerable to corrosion.

Do not leave fasteners exposed to the environment if at all possible. However just a short distance away roofing screws that are fixing a galvanised sheet to a zinc sheet are obviously corroding due to being in contact with the dissimilar roof sheets. Zincalume steel in contact with or receiving of run off water from lead is prone to corrosion. Whether a screw is galvanized or.

The primary metals of concern in terms of galvanic corrosion contact with copper are aluminum and zinc. Therefore zinc serves as a protective barrier for screws. Large differences in the thermal expansion rate between copper and these metals may cause additional problems. Zinc electroplated screws without additional coatings are not for use in exterior applications and will corrode quickly.

Also avoid using copper and some brass fasteners with aluminum zinc and most ferrous metal.