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

  • Writer: David Bennett
    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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