Effects of Calcareous Deposits on Water-Lubricated Propeller Shaft Operations
Authors: Sheridan Trisha 
Published: October 2023

Calcareous deposits in some locations are an ongoing problem in water-lubricated propeller shaft applications causing accelerated bearing wear. Literature has shown deposit formation is correlated to Impressed Current Cathodic Protection (ICCP) system operations, but as ICCP is an essential system for corrosion control of marine vessels, alternative solutions must be found to eliminate the issue. Testing at Thordon Bearings Inc. included 5 metals – CuSn12 bronze (CC483K equivalent), PN-91 bronze (C90500 equivalent), Inconel 625®, 316L SS and Duplex 2205 and two applied voltage settings representing protected conditions (-850 mV SCE) and over-protected conditions with hydrogen evolution (-1150 mV SCE) in artificial seawater. It also included a subset of samples with PN-91 liners, coating on the C35E mild steel rod, an applied voltage of -850 mV SCE, run in 3 types of water; artificial seawater, softer fresh water and hard fresh water. Testing resulted in the following outcomes:

1. Calcareous deposits formed on all metal surfaces of uncoated samples tested in artificial seawater at both applied voltages (-850 mV and -1150 mV SCE), at relatively indistinguishable quantities, though deposits appeared less strongly adhered to Inconel 625®.

2. Deposits consisted mainly of aragonite (calcium carbonate) in all liner materials, but slight differences were noted for each liner type. Bronze liner deposits also included minor amounts of copper and calcite. Stainless steel liners showed calcium carbonate (likely aragonite) and lesser amounts of ha lite sodium and magnesium, presumably as halite (sodium chloride) and brucite (magnesium hydroxide). Inconel 625® liners in particular showed a significant increase in brucite and halite, leading to a weaker more porous structure which could explain field observations noting improved resistance to problematic calcareous deposit growth.

3. Overall deposit formation was approximately 3x higher on uncoated samples tested in over-protected conditions vs protected conditions, however, liner deposits were approximately 2x lower on the same samples. It is unknown why the deposits were lower on the over-protected samples, but it is theorized that it may be due to selective deposition in the presence of multiple meta ls (an untested theory).

4. Testing of a coated sample in artificial seawater at -850 mV SCE yielded less than 1.5x liner deposits (in grams) after 4 weeks of testing as compared to uncoated seawater liner deposits after 2 weeks of testing. The deposition rate was slower for coated samples, as expected. The composition of deposits appeared unchanged compared to uncoated samples tested.

5. Testing of a coated sample in hard fresh water at -850 mV SCE showed a significant quantity of precipitated particles suspended in the water, deposited on tank walls and covered the entire submerged area of the test specimen. The quantity of liner deposits from the hard water sample was less than the coated seawater sample.

6. Testing of a coated sample in softer fresh water at -850 mV SCE showed a reduction of deposits as compared to uncoated seawater samples and no deposits were noted on tank surfaces.

7. The prominent factors affecting calcareous deposit formation from testing and research publications were determined to be applied voltage, mineral content of the water/salinity, temperature, pH, time and liner material selection (i.e., Inconel 625® or equivalent). The most easily manipulated factors that could be used to provide a solution are applied voltage, mineral content of the water and liner material selection; multiple mitigation efforts are presented.