What is Corrosion?

When it comes to Shipbuilding and repair, one could argue that corrosion starts at the same moment as the design and construction of the vessel, as materials are prepared to do the job.

There's a permanent corrosive action, under meteorological conditions, mild or severe, on submerged parts of a ship or structure.

Simply put, corrosion is the destruction of a material caused by a chemical or electrochemical reaction with its environment.

This interaction between different materials and the environment is influenced by the different electrical potentials of the metallic parts, temperatures, concentrations, etc and the result is corrosion - basically the deterioration on the metal's surface due to electrical, chemical or mechanical events that occurs.

When two dissimilar metals (electrodes) such as zinc and steel, are immersed in a conductive liquid acting as an electrolyte - sea water, for example - an electrical potential difference between these two materials (electrodes) occurs. This conjunction of materials can be called a cell.

An electrical current flows from the so called noble element (in this case steel) to the less noble metal (Zinc).

In this particular cell, the less noble metal (Zinc) is called the anode and the more noble metal (steel) is called the cathode. On the surface of both, electrochemical reactions take place.

The most important agent of corrosion between the two metals in contact is the electrical potential, which depends on the natural values of the anode and the cathode, given that the anode is attacked and dissolved and the cathode remains protected or unaffected.

The anode (Zinc) slowly dissolves in the electrolyte while it protects the cathode (steel) against corrosion.

This method of protection is called cathodic protection (CP) by sacrificial anodes.

Cathodic Protection (CP) in fact, uses the addition of direct current towards metallic objects to shift the object potential to values where corrosion is effectively suppressed.

Generally, CP is seen and applied as a secondary method to protect against corrosion.

In order to obtain a good protection in vessels and structures, it's absolutely necessary to apply a primary corrosion protection, provided by a good quality adhesive coating (Paint on ships, mainly).

By using a proper coating, the required protection current for CP is reduced to small values.

Nevertheless, part of the surface will be affected and left without paint due to collisions against docks, tugs, defenses, anchors, etc, which will cause degradation in the protection.
Next, the diffusion of water and oxygen in these areas of the submerged steel will ignite the rusting process.

The corrosion protection with Zinc anodes increases specifically where the coating deteriorates and will safeguard from corrosion the steel surfaces.

However, cathodic protection under a disbanded coating is not possible. (Never paint over the anodes).

Zinc-it anodes quality is certified by the ISO 9001:2008 process trough the expected parameters and by experienced staff.

The use of pure raw materials and alloy composition complies with the worldwide accepted standard - MIL-A-18001 – and final quality checks, weight, packaging and delivery make a difference towards our clients.

Zinc-it anodes are highly valued with excellent quality records appreciated by Shipbuilders and Yards, corrosion consultants and equipment retailers all around Europe.
All shipments are safeguarded by the chemical analysis issued to each production batch. A typical example can be seen here.

Work against corrosion – Materials and applications

Materials generally applied as sacrificial anodes are:
» Zinc and aluminium alloys for offshore installations – Ships / Rigs / Sea Pipelines
» Magnesium and zinc alloys for land installations – boats on sweet-water / Land Pipelines

Although aluminium is also used, the most used anodes are made of Zinc, which doesn't require a tight control and delivers a continuous and efficient current.

An imperative for this to work is the composition that must be exactly to the specifications. They are used on a large range of situations, not only on Ships. You can find them on:
» Engines and coolers
» Submarine pipes
» Buoys
» Chains
» Tanks
» Condensers
» Cisterns
» Metal towers, bridge's foundations

When is cathodic protection required?

The application of cathodic protection on metallic objects depends mainly on the nature of the soil or water in which the object is buried or immersed. Generally speaking, cathodic protection is required (among other situations) when:
» Objects are battered by stray currents from other DC-sources
» Two different metals that are in some way connected to each other resulting in a corrosion cell

Positioning of the sacrificial anodes

Anodes are produced in different shapes and sizes, depending on the size of the areas to protect. Metallic parts of the vessel must be in contact with an anode for which bolts are used or connection cables or simply welded to the steel.

On the external part of the vessel, the anodes must always be placed on a parallel position on the longitudinal length of the ship, thus obtaining the maximum performance of CP and keeping the drag on the ship's movement to a minimum.

"Zinc-it" anodes are free of any rough edges or square angles, precisely to avoid drag increase.

The anodes are to be installed along the wet surface of the hull, above and below the balance keels, and in higher number around the stern due to the high density of current originated by the propeller's movement.

The intake tanks must be protected as well using anodes of our NT series.

CP protection on a ship is obtained with Zinc anodes applied to protect the following external and internal parts:
» Stern, rudder and axle
» Hull
» Boxes and sea valves
» Propellers
» Ballast tanks
» Machinery, hydraulic groups, etc

Amongst the elements which require special protection are:
» Propeller and Transmission shaft
» Metallic rudders
» Metallic keels
» Flaps of motor boats

Calculating your CP needs

Although every new Ship or Vessel is always fitted with corrosion anodes calculated specifically for each project (dimensioned by internal or external corrosion consultants) it often happens that during the life usage of the Ship, there simply isn't info available or a consultant that can help and therefore some calculations have to be made for replacing or maintenance.

On a simple manner, one can roughly calculate the anodes needed for protecting a certain area.

But again, this should be made by a corrosion consultant to get the most accurate results.

A few factors that go into the equation that any ship responsible must be aware of.

Density of the current
The electrochemical and mechanical conditions are - as well as temperature, salinity, drag ratios, etc - important factors when it comes to calculate a specific protection system.
Such system is usually expressed by the density of the electrical current required to provide enough or sufficient potential to the surface we wish to protect.
The normal density of the current for hulls is usually in the range of 10 mA/m2 to 30 mA/m2, although these are not rigid norms and must be adapted in specific circumstances and checked for every installation.

Life of the anode
A Zinc anode is set for duration of approximately 2 to 3 years and usually they are replaced when the ship comes to dry dock for inspection, around these typical periods of time.

How many anodes?
We know the following values;
The Potential has a value of 1.05 V, the Capacity (Amp/Hour per Kg) has a value of 780 and Efficiency is of 95 %
Some simple calculation can be made to get a first glimpse of what your protection needs are and can be obtained by using these well established formulas:

These are values for areas of plain uncoated steel As the areas receive a first line of protection, such as paint, the obtained value will on average, be cut by roughly 30%.

Anodes must be necessarily replaced when they get around 50 to 30 % of their original
weight/size or as they show signs of significant wear.

The Always's and Never's of the Zinc Anodes

» Change your anodes when they are 50% corroded.
» Make sure they make good electrical contact – REMOVE ANY PAINT and clean the mounting surface
» On bolted anodes be sure to use news fasteners every time – even stainless bolts will probably fail as result of corrosion.
» Keep the protected areas immersed in water so that the anodes can work

» Paint over the anodes. They will be isolated and simply will not work
» Mix anode types –aluminium anodes will tend to protect the zinc anodes in the same bonding circuit
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