Understanding Corrosion

Understanding Corrosion

Corrosion is not a single process. Several distinct mechanisms fall under the general term "rust," each with a different cause, a different visual signature, and a different required response. Treating all corrosion identically is a common reason a minor, containable issue is allowed to progress into a structural one.

Uniform corrosion 

is even, surface-level rust spreading gradually across exposed, unprotected steel. It is the most visually apparent form and, as a result, the easiest to identify and address early.

Galvanic corrosion 

occurs when two dissimilar metals are in direct contact in the presence of moisture. One metal corrodes preferentially to protect the other, which accelerates corrosion specifically at the contact point rather than across the surface generally. This is the reason fasteners and repair hardware must be matched to the base metal rather than selected for convenience — a stainless fastener installed on an untreated steel railing will accelerate corrosion at that exact connection point.

Crevice corrosion 

develops in narrow, poorly ventilated gaps — under fasteners, at overlapping joints, anywhere moisture becomes trapped without airflow. It is frequently underestimated in severity because the visible indication (typically a stain) understates the extent of the corrosion occurring inside the gap itself.

Pitting corrosion 

produces small, localized, deep points of material loss on a surface that otherwise appears intact. It is the most deceptive of the four types, since a structure can retain a sound visual appearance while pitting has already materially reduced wall thickness at isolated points.

Regional factors accelerating corrosion in the GTA

Road salt exposure is the dominant accelerating factor for any structure near a driveway, parking area, or walkway — chloride exposure increases corrosion rate substantially beyond what moisture alone would produce. Freeze-thaw cycling compounds this effect: moisture driven into small cracks and joints expands and contracts through repeated winter cycles, widening existing damage incrementally with each cycle.

Why intervention timing determines outcome and cost

Corrosion is cumulative and progressive. Surface rust identified early is addressed through cleaning and recoating. The same corrosion, left through additional freeze-thaw cycles, progresses to a structural repair, and eventually to full replacement. The maintenance interval selected for a given structure is the primary factor determining which category of cost a property owner incurs.