How chlorides interact with factors like electrical conductivity (EC), pH, carbonates, and bicarbonates, and how these influence corrosion in stainless steels.

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