Understanding Building Material Corrosion & Moisture Damage

Corrosion and Moisture Issues in Building Materials

Localized Corrosion (Pitting)

Pitting: A form of localized corrosion characterized by small cavities on a material’s surface. Pits vary in number, shape, and size. They can significantly contribute to component failures under high stress, and can lead to stress corrosion cracking. Pitting occurs in various alloys and metals, including stainless steels and aluminum alloys.

Stress Corrosion Cracking

Corrosion and Stress Cracking: A failure mode where cracks form in a component due to the combined action of mechanical stress and a corrosive environment (e.g., saline or otherwise aggressive). This affects high-strength metals like stainless steels and aluminum alloys.

Humidity and Its Effects

Humidity: Water and temperature significantly affect the quality and physical behavior of construction materials. Water vapor and temperature gradients *do not* follow a simple linear relationship. Temperature and water vapor migrate through a wall from areas of higher temperature to lower temperature. When warm, moist air encounters a cold surface, condensation occurs. The degree of condensation depends on wall insulation. Concentrated condensation can lead to bacteria and fungi growth, paint blistering, and other issues.

Water Intrusion Pathways

Water can attack a wall in several ways:

• a) From the outside in: Following the law of gravity, water penetrates materials, as they are not completely waterproof.
• b) From the ground up: Water rises through the ground via capillary action. The extent of capillary rise depends on capillary size, air pressure, and height above sea level.
• c) Trapped moisture: Moisture can be retained within building materials during the construction process.

Excess water in buildings can trigger various adverse effects, including biological and chemical processes that lead to cracking, loss of structural capacity, and fracture or loosening of building elements.

Biological Processes and Moisture Damage

• a) DIN (Likely referring to mold growth related to DIN standards): Results from prolonged moisture presence in porous materials like plaster, natural stone, or brick. These materials absorb significant ambient or rainwater, which is retained in poorly ventilated exterior walls without solar radiation.
• b) Efflorescence: Occurs when moisture migrates from the interior of a wall to the exterior. It appears as white patches containing a high concentration of mineral salts (nitrates, sulfates, etc.) from the building materials. These salts crystallize on the surface, creating different textures and appearances.
• c) Black Spots: Colonies of fungi, whose spores float in the air, concentrate on cold, poorly ventilated walls and ceilings. Their appearance and location vary, but they are often associated with condensed moisture on cold stone floors and rising damp.
• d) Material Degradation and Disintegration: Manifests as bulges and bumps on a wall’s surface, or as plaster disintegration. This is caused by moisture within the wall, triggering chemical reactions. Salts react with lime and calcium aluminate in cement, forming expansive compounds that increase in volume. This expansion breaks down the material, pushing surface coatings (like paint) outward. The reaction requires a substantial water supply.