Galvanized iron (GI) sheets are a ubiquitous building material across the Philippines, relied upon for everything from sturdy roofing to durable fences. Their widespread and enduring use is largely attributed to a key, fundamental property: inherent corrosion resistance. This article delves into the fundamental principles of corrosion resistance of these GI sheets in the Philippines, meticulously explaining how they effectively stand up to the relentless tropical elements.
The Composition and Creation of GI Sheets
At its core, a GI sheet begins its journey as a basic carbon steel sheet. Steel, while inherently strong and versatile, is highly susceptible to the destructive process of rust, particularly when extensively exposed to both oxygen and moisture simultaneously. To effectively combat this pervasive vulnerability, the steel undergoes a specialized manufacturing process known as galvanization, which meticulously applies a protective layer of zinc onto its surface.
The Dual Mechanisms of Corrosion Resistance
Barrier Protection
Isolation from Elements
The zinc coating literally physically separates the underlying steel substrate from direct contact with corrosive elements present in the surrounding environment, primarily oxygen and water. As long as this integral zinc layer remains intact and undamaged, the steel is effectively shielded from direct interaction with moisture and atmospheric air, which are the main and indispensable ingredients for the initiation and progression of rust formation.
Formation of Zinc Patina
Upon initial exposure to the ambient atmosphere, the pure zinc surface of a GI sheet promptly reacts with airborne oxygen, ambient moisture, and atmospheric carbon dioxide. This series of chemical reactions rapidly culminates in the formation of a thin, remarkably dense, and highly adherent layer of various zinc corrosion products, which is often collectively referred to as a “patina” (primarily consisting of stable zinc oxide and zinc carbonate compounds).
Sacrificial (Cathodic) Protection
Electrochemical Principle
From an electrochemical standpoint, zinc is inherently more electrochemically active (i.e., less noble) than steel (which is primarily composed of iron). In the unavoidable presence of an electrolyte (such as condensed moisture, rainwater, or humid air), a small but active galvanic cell is spontaneously formed if both the zinc and the steel are exposed simultaneously. In this electrochemical setup, the more active zinc willingly acts as the “sacrificial anode” and, consequently, corrodes preferentially, while the less active steel (which functions as the “cathode”) remains robustly protected from corrosion.
Protecting Scratches and Edges
If the GI sheet happens to be scratched, cut during fabrication, or otherwise damaged, thereby exposing the underlying steel substrate to the elements, the adjacent and still intact zinc coating will continue to actively protect the newly exposed steel. Instead of the steel rusting directly at the damaged point, the surrounding zinc will be consumed and corrode first, effectively sacrificing itself to protect the more valuable and structurally critical base metal.
The Philippine Climate: A Corrosion Challenge
High Humidity and Moisture
The Philippines consistently experiences persistently high levels of atmospheric humidity throughout the entire year. This pervasive moisture translates to more water vapor in the air that can readily condense onto metal surfaces, thereby forming the essential electrolyte necessary for electrochemical corrosion reactions to occur. Even in the absence of direct rainfall, this constant dampness actively promotes the formation and spread of rust.
Heavy Rainfall and Typhoons
The country’s frequent and often intense rainfall events, coupled with the recurring onslaught of powerful typhoons, mean that GI sheets are constantly subjected to direct and prolonged exposure to water. Stagnant or standing water, particularly in hidden crevices or areas where debris accumulates, can significantly accelerate localized corrosion.
Saltwater Exposure (Coastal Areas)
For structures strategically located in coastal regions, such as parts of Paranaque City in Metro Manila situated near the expansive Manila Bay, airborne salt spray carried inland from the ocean poses an exceptionally severe and aggressive threat. Chloride ions present in saltwater are highly aggressive corrosive agents that possess the capacity to penetrate and degrade virtually any protective layer, thereby accelerating both the direct corrosion of the zinc coating and, subsequently, the underlying steel. This demanding environment necessitates enhanced and specialized protective measures.
Enhancing and Maintaining Corrosion Resistance
Thicker Zinc Coatings
A smart investment in GI sheets featuring a demonstrably higher zinc coating mass per unit area (often indicated by a higher coating designation like Z275 instead of Z180, for instance) provides a substantially larger sacrificial layer. This greater reserve of zinc significantly extends the sheet’s life, especially when deployed in aggressive environments prone to rapid corrosion.
Alloy Coatings (e.g., Galvalume)
Advanced materials like Galvalume (a steel sheet expertly coated with a specialized alloy of aluminum, zinc, and silicon) offer superior and often multi-fold improvements in corrosion resistance, particularly when exposed to highly corrosive saltwater and acidic environments, compared to standard galvanized sheets. The crucial aluminum component in the alloy forms a more stable and resilient passive layer, further bolstering protection.
Pre-Painted Galvanized Iron (PPGI)
Factory-applied paint coatings directly onto galvanized sheets provide an additional, durable, and highly effective barrier against environmental elements. This “duplex system” intelligently combines the inherent sacrificial protection offered by the zinc layer with the robust barrier protection provided by the paint, dramatically extending the overall lifespan of the sheet and concurrently offering enhanced aesthetic benefits through a wide range of colors and finishes.
Proper Installation
Ensuring scrupulous attention to proper installation techniques is paramount. This includes correct overlapping of sheets to prevent water ingress, secure fastening with suitable fasteners (e.g., self-drilling screws with durable rubber washers) to create watertight seals, and effective sealing of all joints and penetrations.
Key Takeaway
The remarkable corrosion resistance of GI sheets in the Philippines is a powerful testament to the scientific ingenuity that underpins the galvanization process. The dual and complementary protection mechanisms—namely, the robust physical barrier provided by the zinc coating and the ingenious electrochemical sacrificial action of the zinc actively protecting the underlying steel—make these sheets exceptionally durable and reliable building components.