1.What effect does chemical media have on the corrosion rate of Q355B?
The base material (low alloy steel) or surface protective layer (such as galvanized layer) of Q355B has a very slow corrosion rate in a dry environment at room temperature without chemical media (the annual rust depth of bare steel is about 0.01-0.05mm, and the annual consumption of hot-dip galvanized layer is about 1-3μm); however, under the action of chemical media, the corrosion rate will increase exponentially.
2.What is the corrosion effect of chloride ions on Q355B?
Process: Chloride ions preferentially adsorb on tiny defects on the Q355B surface (such as pores in the galvanized layer and scratches on the steel), penetrating the oxide film and forming "corrosion pits." Low oxygen concentration within the pits (an enclosed environment) creates a micro-battery with an "anode inside the pit and a cathode outside." Fe²⁺ and Cl⁻ within the pits combine to form soluble ferrous chloride (FeCl₂), further accelerating corrosion within the pits and ultimately forming "perforations."
Damage: Pitting corrosion has a corrosion rate 50-100 times that of uniform corrosion and is highly insidious. For example, Q355B galvanized steel pipelines in coastal areas may appear intact on the surface, but internal holes 1-2 mm in diameter may have formed due to pitting, leading to leaks. If used in load-bearing structures (such as steel supports), pitting pits can become stress concentration points, triggering "corrosion fatigue fracture," a risk far greater than that of uniform rust.
3.What will happen when there are gaps in Q355B components?
Process: The chemical medium (such as rainwater containing chloride ions) within the crevice cannot flow. Oxygen is rapidly consumed, forming an "oxic zone" and an "oxygen-rich zone" outside the crevice, creating a micro-battery. Steel within the crevice continuously dissolves, and corrosion products (such as Fe(OH)₂) accumulate, further hindering the flow of the medium and creating a "vicious cycle."
Damage: Crevice corrosion occurs 20-50 times faster than uniform corrosion and often occurs at critical joints. For example, in the bolted joints of Q355B steel trusses, if not sealed, the chloride ions trapped in the crevice can cause bolt rust and seizure within a year, and potentially perforation of the connecting plate within two to three years, leading to structural instability.
4.What effect does it have on corrosion resistance when Q355B is subjected to tensile stress?
Process: Chemical media penetrate tiny cracks in the steel, reducing the surface energy at the crack tips and causing the cracks to continue to propagate under stresses far below the steel's yield strength, ultimately leading to silent fracture.
Damage: This type of damage is sudden and occurs without obvious precursors. For example, Q355B steel storage tanks in chemical industrial parks (which bear internal pressure, i.e., tensile stress) exposed to acidic gases containing SO₂ could suddenly develop stress corrosion cracking after one to two years of use, leading to tank leaks and potential safety incidents. The damage is far greater than that caused by uniform rust, which slowly reduces strength and can be detected early.
5.What is the "core influencing position" of chemical media factors?
The impact of chemical media factors on the corrosion resistance of Q355B is not only an "increase in corrosion rate", but also a "qualitative change in the form of damage" - from "observable and maintainable uniform rust" to "highly hidden and harmful localized corrosion (pitting, crevice corrosion, stress corrosion cracking)". Its impact far exceeds that of ordinary climatic factors and is the primary consideration when applying Q355B in scenarios such as "coastal areas, industrial areas, and chemical industrial zones."