Thursday, 08 December 2011 11:30

Propeller corrosion

Corrosion

Electrolysis is a form of corrosion. This is not to be confused with the breakdown of metal by mechanical means such as wear, galling, erosion, or cavitation. Electrolysis is the alteration, decomposition or breakdown of metals or alloys from persistent electrochemical reactions (or by direct chemical attack).

Simplistically Corrosion can be broken down to two possible causes: Galvanic Action or Electrolysis. It is impossible to tell which was responsible after the fact. electrolysis-3-blade-fixed-propeller

Galvanic Action: This is the most common cause of electrolysis in the marine environment. Galvanic corrosion is the interaction of two dissimilar metals connected in an electrolyte (salt-water). In this situation the least noble becomes the anode and corrodes. The more noble is the cathode which in extreme cases can actually be coated with the anode metal. Metals can be connected through a bonding system or directly such as Bronze prop to Stainless shaft. Thus by adding a shaft zinc the zinc deteriorates before the prop.

Electrolysis on the other hand is corrosion caused by stray current. Generally this is much more aggressive causing a lot of damage in a relatively short amount of time.

Typically when Nickel-Aluminium-Bronze (Nibral) corrodes the aluminium leaches out of the alloy leaving irregular shallow pitting in the blade surface. In advanced cases the blade edges become scalloped.

Manganese Bronze on the other hand turns copper red as the zinc leaches out of the alloy. In advanced cases the blade edges actually begin to peal apart much like the leaves of a book. Prior to this the propeller will cease to ring and only thud when tapped with a hammer.
In copper based alloys once corrosion has set in it becomes almost impossible to weld the propeller. The metal has changed sufficiently that attempting to weld the affected areas only results in creating a larger hole.

Corrosion can be aggravated by cavitation. Cavitation is the mechanical breakdown of the propeller material through the implosion of small air bubbles on the surface of the blades as a result of the water “boiling” from low pressure. If the structure of the metal is compromised by corrosion than cavitation erosion occurs much more readily.

The chromium, nickel, and molybdenum content of stainless steel are major contributors to it’s corrosion resistance. Stainless steel is interesting because it is one of the metals that, under certain conditions becomes more noble. The oxygen in the atmosphere and in moving water is sufficient to allow stainless steel to form or repair a tough transparent film of chromium-oxide that renders the metal non-corrodible. When this film is damaged under conditions where there is insufficient oxygen to repair it stainless becomes active and corrodes freely. Typically this corrosion is localized but can be very aggressive. This type of corrosion is often referred to as crevice corrosion.

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Published in Marine corrosion

Thursday, 08 December 2011 11:24

Corrosion cells on ships

Simple anti corrosion measures.

When steel is immersed in sea water (e.g. a ships hull) small galvanic currents are initiated at anodic areas of the metal surface, causing corrosion. Such corrosion predominates at the stern of a ship, where the combined effects of increased turbulence and differential metals results in accelerated corrosion rates. The application of Cathodic Protection effectively suppresses these corrosion cells by applying an opposing current from external anodes and if the propeller is to receive the benefits of cathodic protection then there must be a continuous electrical circuit between the propeller and the ships structure. This circuit usually exists when the propeller is at rest, where a metal to metal contact is made between the shaft and the stern tube liners, or main engine bearings and journals.
However, whilst the shaft is turning the bearing lubrication creates an intermittent high resistance which effectively insulates the propeller from the hull structure and since the propeller presents a relatively large surface area of bare metal, it attracts cathodic protection currents, which tend to discharge by arcing across the lubrication film and in so doing, results in spark erosion which eventually leads to pitting and ‘striping’ of white metal bearing surfaces. It is generally accepted, that the effects of arcing are minimised when the potential across the shaft/hull interface is less than 50 mV.

Published in Marine corrosion

Wednesday, 07 December 2011 16:53

Principals of marine corrosion

What is galvanic corrosion? Galvanic corrosion or Bimetallic Corrosion or Dissimilar Metal Corrosion, as sometimes called, is defined as the accelerated corrosion of a metal because of an electrical contact (including physical contact) with a more noble metal or nonmetallic conductor (the cathode) in a corrosive electrolyte.

The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect.

The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced, the dissimilar metals do not have to be widely different i.e. copper and steel, two different steel water pipes will produce the same effect.

What causes galvanic corrosion? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. When the corrosion potentials of various metals and alloys are measured in a common electrolyte (e.g. natural seawater) and are listed in an orderly manner (descending or ascending) in a tabular form, a Galvanic Series is created. It should be emphasized that the corrosion potentials must be measured for all metals and alloys in the same electrolyte under the same environmental conditions (temperature, pH, flow rate etc.), otherwise, the potentials are not comparable.

The potential difference (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte will also affect the degree of attack. It is generally accepted that the Mediterranean sea is the saltiest with mid Atlantic and the Caribbean being second saltiest (All the popular yacht cruising grounds) the Arctic is the least salty. The cathode to anode area ratio is directly proportional to the acceleration factor.

How to prevent galvanic corrosion? Galvanic corrosion is best prevented by not putting your vessel in the sea! However this not practicable so some of the following will help.

Select metals/alloys as close together as possible in the galvanic series.
Avoid unfavourable area effect of a small anode and large cathode, this effect is utilised for protection with the typical zinc anodes used on ships where the anode is encouraged to corrode rather than the hull.
Insulate dissimilar metals wherever practical or bond them together to achieve close to zero potential difference a minimum of 16mm² copper cable is needed to achieve low Ω bonding.
Apply coatings with caution. Paint the cathode and keep the coatings in good repair on the cathode and make sure the anode is clean.
Avoid threaded joints for materials far apart in the galvanic series, better still avoid joints with greatly dis-similar metals.

Published in Marine corrosion

Wednesday, 07 December 2011 16:40

Propeller slip rings

GadSolutions manufactures slip rings for propeller shafts to combat the electrolysis action of a spinning shaft (steel) within bearings (usually bronze/white metal) immersed in sea water (electrolyte).

Various combinations of slip-ring and carbon brush materials are available but time has determined that only high silver composition brushes running on a silver track or 0% Oxygen Copper, can provide the effective and sustained low conductivity necessary to ensure that the shaft bonding and its connections maintains a contact resistance no greater than 0.001 Ohms.

Current passing through the gearbox bearings can severely damage them see Bearing degradation through electric current

At 59.6×10⁶ S/m copper has the second highest electrical conductivity of any element, just after silver. This high value is due to virtually all the valence electrons (one per atom) taking part in conduction. The resulting free electrons in the copper amount to a huge charge density of 13.6×10⁹ C/m³. This high charge density is responsible for the rather slow drift velocity of currents in copper cable (drift velocity may be calculated as the ratio of current density to charge density). For instance, at a current density of 5×10⁶ A/m² (typically, the maximum current density present in household wiring and grid distribution) the drift velocity is just a little over ⅓ mm/s.

Voltmeter showing the efficacity of slip rings on propeller shafts. In practice very few ships engineers have ohm meters capable of reading such low values and it is therefore difficult to determine the conductivity of the system. If GadSolutions install the system we use our calibrated low ohm reading test meter to insure a sound electrical connection with all parts of the system. A voltmeter with a range of 75 Millivolt F.S.D is installed to constantly monitor the connections and ensure the level of potential difference between shafts and hull is kept below 50 Millivolt (believed to be the minimum at which electrolysis starts) in practice we usually achieve 3 Millivolt PD.

The system of shaft bonding comprises a split slip-ring arrangement and ancillary brush gear, which is designed to facilitate ease of assembly by proficient technical personnel and without the need for specialist tools. We hone each slip-ring to ensure a precise fit and constant contact with the propeller shaft, coupled with the 4 grub screws that drill into the shaft ensuring a very good electrical connection.

The monitoring panel shows a working installation of the slip-rings, 1 millivolt was shown as the potential difference between propellers ,shafts and the hull. With such a low reading no electrolysis will take place.

The finished article, a gadsolutions slip ring installed on a propeller shaft

brush-gear

Published in Marine corrosion