All areas of the marine industry fight a constant battle against corrosion.
The shipping industry is one that continually faces the corrosion challenges stemming from marine environments, particularly seawater.
Because seawater contains a significant concentration of dissolved salts and is very corrosive to steel, infrastructure and assets in or near marine environments are particularly susceptible to corrosion. Efforts to mitigate corrosion in marine environments continue as industries develop and implement solutions to prevent asset degradation.
The most frequent forms of corrosion found on chemical tankers are uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, and microbiologically influenced corrosion.
The most efficient system for combating underwater corrosion is 'cathodic protection'. The basic principle of this method is that the ship's structure is made cathodic, i.e. the anodic (corrosion) reactions are suppressed by the application of an opposing current and the ship is thereby protected.
CATHODIC PROTECTION USING SACRIFICIAL ANODES: THE BASICS
How does corrosion take place in ships?
Ships are made of steel; whose main component is iron. Iron is an electrochemically positive element, i.e., it has a tendency to give up electrons to become a free ion. Sea water is composed of oxygen and hydrogen, and it produces electrochemically negative hydroxyl ions which can accept the electrons given by Iron. This way the Iron ions combine with the hydroxyl ions of water to form Iron Hydroxide. This is called the oxidization of Iron, and this oxide is what we call as the brown color rust.
The basic idea of using sacrificial anodes is to use a metal like Zinc/Aluminium and create its contact with the surface to be protected.
The simplest picture which comes to mind is simply using a flat bar of the metal and fix it to the surface to be protected. This is actually the method commonly used to protect the outer ship’s hull.
To protect steel successfully using cathodic protection, it is therefore only necessary to lower its potential by around a quarter of one volt (250mV).
Cathodic protection using sacrificial anodes produces a decrease in the potential of the ship by connecting the vessel to a metal which takes up a reversible potential of less than –850mV (S.C.E.) and allowing the sacrificial metal to produce the electrons rather than the corrosion reaction of the steel.
Anode Classification
Anodes can be classified based on their shape, size, material, mounting method and method of securing to the surface to be protected.
The following are some widely used shapes for anodes:
Flat or block shaped
Cylindrical or semi-cylindrical
Tear-drop anodes
Bracelet anodes
Disc shaped
Tubular anodes
Anodes can be of different shapes based on their applicability. The selection of the shape of anode depends on several factors. Some of these factors are:
shape of the surface to be protected,
availability of space,
accessibility,
ease of installation
special considerations, e.g., effect on resistance for small boats
For example, flat anodes are used mostly for flat, large surfaces like the ship’s hull. Tear-drop anodes are used in high speed boats where streamlining of water is important as flat anodes will increase the boat’s resistance. Bracelet anodes are used for pipelines and propeller shaft, while tubular anodes are used for cables. There are no fixed rules here though, and the choice depends on the availability, cost and flexibility in design. For example, cylindrical anodes can also be used to protect pipelines, and it is not necessary to use bracelet anodes if they are costlier.
Anode Material
Usually for marine applications, Zinc or Aluminium anodes are deployed. Zinc has been traditionally used for corrosion protection, though Aluminium is now widely used. The two properties which measure performance of an anode are listed below.
1. Closed Circuit Potential – the first parameter, Closed Circuit Potential signifies the ease with which the anode will be corroded. The more negative the value, the more readily the anode will get corroded. Generally, a potential of less than -0.08 Volts is required for cathodic protection of shipbuilding steel to be effective.
2. Electrochemical Capacity (in Amp-hr/kg) – The second parameter, the Electrochemical Capacity, signifies the rate at which the anode material will be consumed.
The two parameters for Zinc and Aluminium are listed in the table below:
Properties of Anode Materials (Source: DNV RP-B401)
We can see from the above table that Aluminium has a higher closed circuit potential – so it will more readily start working compared to Zinc. It also has higher Electro-chemical capacity compared to Zinc, and will be longer lasting for the same anode size.
Further, in fresh water application, Zinc tends to develop a calcareous coating on the anode surface, which prevents their effective working.
However, Zinc anodes have sometimes been found more reliable in environments with low oxygen, e.g., marine sediments or areas with high bacterial activity. Thus, while Aluminium is the more efficient one, Zinc may be more effective in some cases.
Further, Aluminium anodes, if falling from a height on oxidized steel, can create sparks. Thus they are not recommended to be used inside cargo tanks of tankers. The maximum height above tank bottom which they must be placed is 28/W meters, where W is the weight of the anode in kgs.
Hence, the selection of the material depends on the type of environment it is going to be used, and should be carefully carried out.
Anode Mounting Method
The next important consideration for installation of anodes is the mounting method, i.e., the configuration of the tubular insert, and the positioning of the anode vis-à-vis the surface to be protected.
Based on mounting technique, there are two major types of anodes which are used in ships:
1. Flush mounted anodes – in this type of anode, the anode material (Aluminium or Zinc) is in direct contact with the surface to be protected. The insert is generally a flat bar which can be welded or bolted to the surface.
Flush Mounted Anode
2. Slender stand-off anodes – In these types of anodes, the anode material is not in direct contact with the surface to be protected, and there is a gap (hence the name stand-off). The insert is generally a tubular one which can be welded or bolted to the surface.)
The benefit of a stand-off design is that it is a more compact design, and the anode material is better utilized in a stand-off design. This is quantified by a parameter called ‘anode utilization factor’. This is the fraction of the anode material which is actually utilized over the lifetime of the anode. For flush anodes, this is around 80%, while for stand-off anodes it is 85 to 90%. Thus, stand-off anodes are better utilized over their lifetime.
Further, in case of flush anodes, due to constant contact between the anode material and the surface, the surface may suffer from embrittlement caused by deposition of ions from the anode material to the cathode (the protected surface).
That said, stand-off anodes protrude from the surface on which they are installed. When used on external hull of a vessel, these affect the streamlined shape of the vessel, and lead to increased drag and higher powering requirements. In comparison, flush anodes are closer and more compliant to the vessel’s geometric shape and have lower effect on resistance. Thus, flush anodes are usually preferred on outer hull due to their low drag properties.
Both Flush mounted and slender stand-off anodes are further classified into Short and Long, depending on their ratio of length to width. The length affects the resistivity of the anode and thus its current capacity.
References:
https://thenavalarch.com/ship-corrosion-cathodic-protection-sacrificial-anodes/