In marine cylinder oil (MCO) formulation, overbased Mg sulfonate is almost always used in conjunction with overbased Ca sulfonate - the combination leverages Ca²⁺'s superior film-forming/rust-inhibition with Mg²⁺'s superior TBN-per-ash efficiency. A typical commercial MCO for VLSFO service (target BN 40–70 mgKOH/g) might use 8–12 wt% Overbased Ca Sulfonate (TBN 450) for primary rust protection and film formation, combined with 5–10 wt% Overbased Mg Sulfonate (TBN 350) to deliver supplementary TBN with lower ash contribution - achieving the target BN at lower total sulphated ash than a Ca-only formulation would require.

ACEA E9 (low-ash Euro VI HDEO for DPF-equipped trucks) and API FA-4 (fuel-efficient low-viscosity HDEO) limit finished oil sulphated ash to ≤1.0 wt%. At the TBN targets required for 60,000–100,000 km drain intervals (finished oil TBN 12–18 mgKOH/g), achieving both high TBN and low S/A requires the formulator to maximise Mg sulfonate and minimise Ca sulfonate in the detergent package. Overbased Mg Sulfonate is the preferred TBN delivery component for these tight ash-budget formulations - it frees ash headroom for ZDDP's antiwear contribution (essential for DPF-equipped engines using API FA-4 low-viscosity grades) and for borated dispersants' supplementary TBN.

Gas engine oils must provide TBN for nitric acid neutralisation (from NOₓ blow-by) while maintaining oxidative stability under lean-burn conditions. Mg sulfonate is well-suited here because its lower ash contribution allows achieving the required finished oil TBN of 8–15 mgKOH/g without excessive sulphated ash in gas engines where deposit sensitivity is a concern. For high-temperature compressor and turbine oils requiring long-drain stability combined with modest acid neutralisation, low-treat-rate Mg sulfonate provides detergency and alkalinity supplementation at minimal viscosity penalty.

Overbased Mg sulfonate is used in neat metalworking fluids and rust-preventive oils for its combined emulsification, corrosion inhibition, and mild alkalinity. In rolling mill oils and neat cutting fluids, the sulfonate surfactant reduces interfacial tension between the oil and metal workpiece, improving surface wetting and lubrication film formation. The alkalinity reserve buffers acidic contaminants from machining operations. Mg sulfonate is preferred over Ca sulfonate in some metalworking applications where calcium soap formation on workpiece surfaces must be avoided.

Q: Why does magnesium sulfonate deliver more TBN per unit sulphated ash than calcium sulfonate?

The difference is atomic weight. TBN is measured in milligrams of KOH equivalent per gram - it reflects the moles of alkalinity per unit mass. Magnesium (MW 24.3 g/mol) is 60% lighter than calcium (MW 40.1 g/mol). When both form carbonates (MgCO₃ vs CaCO₃), each mole of Mg²⁺ contributes the same 2 moles of base equivalents as Ca²⁺ - but per gram, Mg contributes 40.1/24.3 = 1.65× more moles of TBN than Ca. However, MgO (MW 40.3) is only slightly lighter than CaO (MW 56.1), so the sulphated ash conversion factor is similar for both (Mg× 3.32 vs Ca × 3.40). The net result: at the same TBN and treat rate, Mg sulfonate contributes ~38–42% less sulphated ash to the finished oil than Ca sulfonate - a significant formulation advantage in ash-constrained specifications.

Q: Can I use Mg sulfonate as a direct substitute for Ca sulfonate in my existing formulation?

Not as a drop-in one-for-one substitute without bench evaluation. While Mg sulfonate is chemically compatible with Ca sulfonate and all standard co-additives, a direct replacement changes the finished oil's: (1) total sulphated ash - lower with Mg; (2) rust inhibition performance - Ca sulfonate provides a stronger film on iron surfaces; (3) viscosity and density of the blend. In practice, most formulators partial-substitute: replace 30–60% of the Ca sulfonate TBN contribution with Mg sulfonate, then verify the full finished oil against specification tests (ASTM D2896, D874, D130, and any engine performance tests required by the target spec). Contact Sinolook's technical team for guidance specific to your current formulation.

Q: Is Mg sulfonate suitable for ACEA C-sequence (low-SAPS) passenger car engine oils?

Yes - Mg sulfonate is a useful supplementary TBN component in ACEA C3/C5 (S/A ≤0.8 wt%) and C1/C2 (S/A ≤0.5 wt%) formulations, but the primary detergent should still be Low TBN Ca Sulfonate. At very small treat rates (0.3–0.8 wt%), Mg sulfonate contributes supplementary TBN of 0.9–2.4 mgKOH/g while adding only 0.08–0.22 wt% S/A - a reasonable trade-off where the ash budget allows. However, the primary low-SAPS PCMO detergent strategy relies on Low TBN Ca Sulfonate as the base detergent; Mg sulfonate serves as a supplementary TBN booster where needed.

Q: Does overbased Mg sulfonate have any disadvantages vs Ca sulfonate?

The main practical differences are: (1) Weaker rust inhibition - Ca²⁺ adsorbs more strongly to iron oxide surfaces than Mg²⁺, so Ca sulfonate provides superior rust-preventive film formation; Mg sulfonate alone is generally not recommended where rust protection is the primary function. (2) Narrower supply base - Ca sulfonate is produced in larger commercial volumes globally, so Mg sulfonate may have slightly longer lead times in some markets. (3) Less commonly available in Low/Medium TBN grades - most commercial Mg sulfonate is produced at High/Overbased TBN levels; for low-ash PCMO applications requiring low-TBN detergents, Ca sulfonate remains the standard. Sinolook supplies both Ca and Mg sulfonate across all TBN grades to support optimal Ca/Mg blend formulation.

Related: Overbased Calcium Sulfonate · High TBN Calcium Sulfonate · Overbased Barium Sulfonate · Zinc Sulfonate Complex · Primary ZDDP · Borated PIB Bis-Succinimide Dispersant