How chlorides interact with factors like electrical conductivity (EC), pH, carbonates, and bicarbonates, and how these influence corrosion in stainless steels.
- David Bennett
- Apr 3
- 2 min read
Here’s a breakdown:
1. Chlorides and Their Role in Corrosion
Chloride ions (Cl⁻) are aggressive species that can cause pitting corrosion and stress corrosion cracking (SCC) in stainless steel.
They break down the passive chromium oxide layer that protects stainless steel, leading to localized attack.
The risk of chloride-induced corrosion depends on chloride concentration, temperature, and oxygen availability.
2. Relationship with EC, pH, Carbonates, and Bicarbonates
Electrical Conductivity (EC):
A higher EC indicates higher dissolved salts (including chlorides), which increases corrosion risk.
Stainless steel corrosion rates increase as EC rises because more aggressive ions are present in solution.
pH Influence:
Low pH (acidic conditions): Accelerates chloride attack due to increased availability of H⁺ ions that weaken passive films.
Neutral to slightly alkaline (pH 7-9): Still risky if chloride concentration is high.
High pH (>9): Can reduce chloride aggression if carbonates/bicarbonates are present, as they help buffer and form protective layers.
Carbonates & Bicarbonates:
Can be beneficial as they tend to form protective calcium carbonate (CaCO₃) layers that reduce stainless steel corrosion.
However, high bicarbonate levels in the presence of chlorides can still lead to localized attack.
3. Stainless Steel Grade Selection Based on Chloride Exposure
304 Stainless Steel:
Poor chloride resistance, prone to pitting and SCC in chloride-rich environments.
Avoid in seawater, brine, and environments with >200 ppm Cl⁻.
316 Stainless Steel (Mo-added):
Better chloride resistance due to molybdenum (Mo) addition.
Can handle moderate chloride levels (around 1000 ppm Cl⁻), but still not ideal for seawater.
Duplex Stainless Steels (2205, 2507, etc.):
Higher pitting resistance equivalent number (PREN) than 316.
Good resistance to chloride-induced SCC, making them suitable for environments with up to 10,000 ppm Cl⁻.
Super Duplex (2507) & High-Alloy Austenitics (904L, AL-6XN):
Excellent chloride resistance, suitable for aggressive seawater conditions (>20,000 ppm Cl⁻).
Used in desalination plants, offshore structures, and chemical processing.
Ansari, T.Q., Luo, JL. & Shi, SQ. Modeling the effect of insoluble corrosion products on pitting corrosion kinetics of metals. npj Mater Degrad 3, 28 (2019). https://doi.org/10.1038/s41529-019-0090-5
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