Types of Marine Corrosion (Part 1)

Metal parts underwater are subjected to two basic types of corrosion: galvanic corrosion and stray current corrosion.

To best describe corrosion, let’s start with the most common type, rust. We all know rust, but to understand rust, we have to go back to the very beginning. Iron ore has a chemical composition of two iron atoms bonded with three oxygen atoms. As it is mined out of the ground, it’s a brownish-red powder useless to us. But by refining, purifying, and smelting, we create iron, which is useful. We can use it as plain iron, or we can process it further and combine it with other elements to get different types of steel.

Electrochemical Reactions

Iron left out in the rain results in a specific kind of corrosion. It’s called an electrochemical reaction, meaning there is an electrical change. Here’s how that works:

For two iron atoms to really interlock with three oxygen atoms and make iron, they have to share some electrons, which releases a few electrons. Since electricity is just a flow of electrons, those free electrons become a little bit of electricity when the chemical change takes place.

Remember the iron wants to corrode into iron oxide because that is its natural, most stable state. And all it needs for this to take place is oxygen. Water is a supply of oxygen, so iron rusts fastest when it gets wet. You knew that already but now you know why. And that same scenario applies to aluminum and aluminum oxide. Those are the deep, dark secrets of corrosion as they apply to metals. Those are also the basics of an electrochemical reaction, which is known as galvanic corrosion. All galvanic corrosion is an electrical reaction. Not all electrochemical reactions, however, are galvanic corrosion.

Galvanic Corrosion

Galvanic corrosion is an electrochemical reaction between two or more different metals. The metals must be different because one must be more chemically active (or less stable) than the others for a reaction to take place. When we talk about galvanic corrosion, we’re talking about electrical exchange. All metals have electrical potential because all atoms have electrons, which have an electrochemical charge.

Galvanic corrosion of the more chemically active metal can occur whenever two or more dissimilar metals that are "grounded" (connected by actually touching each other, or through a wire or metal part) are immersed in a conductive solution (any liquid that can transfer electricity). Anything but pure water is conductive. Saltwater, freshwater with high mineral content, and polluted freshwater are very conductive, and conductivity goes up with water temperature. That’s one reason why boats in Florida experience more corrosion than boats in Maine.

The simplest example of galvanic corrosion, and the most applicable, is an aluminum lower unit with a stainless steel propeller. The aluminum is the more chemically active metal (the anode), and the stainless steel is the less chemically active metal (the cathode). Several things happen at the same time:

At the Anode

1. Electrons flow from the anode, the metal that is more chemically active (the aluminum drive unit), via the external conducting path to the cathode, the metal that is less chemically active (the stainless steel prop).

2. When this happens, the more chemically active metal atoms become ions (an atom with one or more electrons either missing or added) and break away into the water, where they can bond to oxygen ions, with which they can share electrons and produce aluminum oxide. This is the same process iron ions go through when combining with oxygen ions in water to form iron oxide.

3. The newly formed aluminum oxide molecules either drift away in the water or settle on the surface of the aluminum. Your lower unit is literally dissolving through galvanic corrosion.

At the Cathode

1. Electrons are accepted from the anode; however, they cannot simply accumulate, they react with ions in the electrolyte.

2. The resulting hydroxide ion is alkaline, and makes the electrolyte alkaline in the area of the cathode. This detail is especially important for wooden boats, as an alkaline solution will attack cellulose (i.e. wood).

It's important to understand that for each positive metallic ion released at the anode, electrons in the cathode react to form a negative ion in the electrolyte. Electrically the anodic and cathodic reactions must be equivalent. Increases or decreases in the rate of the cathodic reaction will have a corresponding increase or decrease on the anodic reaction. This is a basic fact in understanding and controlling corrosion. This fact can also be demonstrated by the effect of size ratios between anodes and cathodes. If there is a very large anode connected to a small cathode, the anode will corrode very slowly. However, if a very large cathode is connected to a small anode, the anode will corrode very rapidly. Marine drive components have many aluminum parts. If you do not control galvanic corrosion, over time the aluminum will corrode away.

Galvanic corrosion can also occur without any stainless steel components on your boat. For example, you have an aluminum drive unit and an aluminum propeller, but you dock at a pier with steel pilings or a steel seawall, then plug into shorepower. The ground wire, which is grounded, connects your aluminum components with the submerged steel because the steel is also grounded. Considering the mass of a seawall or even a single piling, your drive and propeller can sustain serious damage. This damage could be prevented with a galvanic isolator.

What to Look For

The first sign of galvanic corrosion is paint blistering (starting on sharp edges) below the water line—a white powdery substance forms on the exposed metal areas. As the corrosion continues, the exposed metal areas will become deeply pitted, as the metal is actually eaten away.

Typical signs of corrosion on marine lower drive units and propellers include blistering paint and the formation of a white powdery substance on the exposed metal areas

Galvanic corrosion of aluminum drive units—or any underwater aluminum on your boat—is accelerated by attaching stainless steel components like propellers, trim planes (if connected to engine ground), and aftermarket steering aids. In doing this, you have introduced a dissimilar metal to which electrons from your drive unit will follow. Another condition that will increase the speed or intensity of galvanic corrosion is the removal or reduction in surface area of sacrificial anodes. But you don’t need stainless steel components for galvanic corrosion to take place. Galvanic corrosion continually affects all underwater aluminum, but at a reduced rate when no dissimilar metals are connected to your aluminum parts. When in contact with an electrolyte, most metals form small anodes and cathodes on their surfaces due to such things as alloy segregation, impurities, or cold working.

We have used stainless steel (cathode) and aluminum (anode) in this discussion as an example, however other metals coupled with aluminum also produce galvanic corrosion cells. For example, zinc connected to aluminum will form a corrosion cell, but in this case, the aluminum becomes the cathode and the zinc (anode) corrodes. One of the worst couples with an aluminum drive would be connecting it with copper or a copper alloy (bronze). Another cause of galvanic corrosion is the shorepower hookup. When you plug in, you tie your aluminum drive unit to other boats using shorepower through the green grounding lead. Your aluminum drive unit is now part of a large galvanic cell (a battery) interconnected with onshore metal that is in the water—as well as other boats—and corrosion may be greatly accelerated.

Article courtesy of Quicksilver Marine.

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Classification of corrosion protection methods (Part 4) - VCI Method, Advantages and Disadvantages

VCI (Volatile Corrosion Inhibitor) method

Mode of action and use


Inhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered the opposite to catalysts, which enable or accelerate certain reactions.

Unlike the protective coating method, the VCI method is an active corrosion protection method, as chemical corrosion processes are actively influenced by inhibitors.

In simple terms, the mode of action (see Figure 1) is as follows: due to its evaporation properties, the VCI substance (applied onto paper, cardboard, film or foam supports or in a powder, spray or oil formulation) passes relatively continuously into the gas phase and is deposited as a film onto the item to be protected (metal surfaces). This change of state proceeds largely independently of ordinary temperatures or humidity levels. Its attraction to metal surfaces is stronger than that of water molecules, resulting in the formation of a continuous protective layer between the metal surface and the surrounding atmosphere which means that the water vapor in the atmosphere is kept away from the metal surface, so preventing any corrosion. VCI molecules are, however, also capable of passing through pre-existing films of water on metal surfaces, so displacing water from the surface. The presence of the VCI inhibits the electrochemical processes which result in corrosion, suppressing either the anodic or cathodic half-reactions. Under certain circumstances, the period of action may extend to two years.

Figure 1: Mode of action of VCI

The mode of action dictates how VCI materials are used. At item to be protected is, for example, wrapped in VCI paper. The metallic surfaces of the item should be as clean as possible to ensure the effectiveness of the method. The VCI material should be no further than 30 cm away from the item to be protected. Approximately 40 g of active substances should be allowed per 1 m³ of air volume. It is advisable to secure this volume in such a manner that the gas is not continuously removed from the package due to air movement. This can be achieved by ensuring that the container is as well sealed as possible, but airtight heat sealing, as in the desiccant method, is not required.

The VCI method is primarily used for articles made from carbon steel, stainless steel, cast iron, galvanized steel, nickel, chromium, aluminium and copper. The protective action provided and compatibility issues must be checked with the manufacturer.

N.B.: The use of water-miscible, water-mixed and water-immiscible corrosion protection agents, corrosion protection greases and waxes, volatile corrosion inhibitors (VCI) and materials from which volatile corrosion inhibitors may be released (e.g. VCI paper, VCI films, VCI foam, VCI powder, VCI packaging, VCI oils) is governed by the German Technical Regulations for Hazardous Substances, TRGS 615 "Restrictions on the use of corrosion protection agents which may give rise to N-nitrosamines during use".


Comparison of advantages and disadvantages of the VCI method

Advantages

	Since the gas also penetrates holes and cavities, these areas also receive adequate protection.
	The period of action may extend to two years.
	The wrapping need not be provided with an airtight heat seal.
	On completion of transport, the packaged item need not be cleaned, but is immediately available.

Disadvantages

	The VCI method is not suitable for all metals. It may cause considerable damage to nonmetallic articles (plastics etc.).
	Most VCI active substances may present a hazard to health, so it is advisable to have their harmlessness confirmed by the manufacturer and to obtain instructions for use.

http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Classification of corrosion protection methods (Part 3) - Desiccant Method, Advantages and Disadvantages

Desiccant method


Introduction

According to DIN 55 473, the purpose of using desiccants is as follows: "desiccant bags are intended to protect the package contents from humidity during transport and storage in order to prevent corrosion, mold growth and the like."

The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecular sieves etc.. Due to the absorbency of the desiccants, humidity in the atmosphere of the package may be reduced, so eliminating the risk of corrosion. Since absorbency is finite, this method is only possible if the package contents are enclosed in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate-controlled or sealed package. If the barrier layer is not impermeable to water vapor, further water vapor may enter from outside such that the desiccant bags are relatively quickly saturated, without the relative humidity in the package being reduced.

Desiccants are commercially available in desiccant units. According to DIN 55 473:

"A desiccant unit is the quantity of desiccant which, at equilibrium with air at 23 ± 2°C, adsorbs the following quantities of water vapor:

	min. 3.0 g at 20% relative humidity
	min. 6.0 g at 40% relative humidity

The number of desiccant units is a measure of the adsorption capacity of the desiccant bag."

Desiccants are supplied in bags of 1/6, 1/3, 1/2, 1, 2, 4, 8, 16, 32 or 80 units. They are available in low-dusting and dust-tight forms. The latter are used if the package contents have particular requirements in this respect.

Barrier films

Barrier films are available in various forms, for example as a polyethylene film or as a composite films with two outer polyethylene layers and an aluminum core. The composite film performs far better with regard to water vapor permeability (WVP), achieving WVP values of below 0.1 (g/m²d). In the composite film, the barrier layers are arranged so as to bring about a considerable reduction in permeability in comparison with a single layer.

In accordance with current DIN standards, water vapor permeability is always stated for both 20°C and 40°C. According to information from the manufacturer, it may be concluded that water vapor permeability rises with increasing temperature and falls with increasing thickness. This problem occurs most particularly with polyethylene films, while aluminum composite films are largely insensitive to rises in temperature.


Placement of desiccant bags

The desiccants should be suspended from strings in the upper part of the climate-controlled package to ensure good air circulation around them.

It is essential to avoid direct contact between the desiccant bag and the package contents as the moist desiccant would promote corrosion.

It is advisable to use numerous small bags rather than fewer large ones, as this increases the available surface area of the desiccant and so improves adsorption of the water.

In order to ensure the longest possible duration of protection, the barrier film must be heat sealed immediately once the desiccant bags have been inserted.

Desiccant bags are always supplied in certain basic package sizes which, depending upon the desiccant unit size, may contain a single bag (of 80 units) or up to 100 bags (of 1/6 unit). The basic outer package should only be opened directly before removal of a bag and must immediately be heat sealed again.


Comparison of advantages and disadvantages of the desiccant method

Advantages

	Desiccants provide excellent corrosion protection to both metallic and nonmetallic items
	Removal of the desiccant on delivery to the receiver is straightforward, unlike the removal of protective films in the protective coating method. The package contents are immediately available.
	No particular occupational hygiene requirements apply as the desiccant is nonhazardous.

Disadvantages

	Placement of the desiccant bags and heat sealing of the barrier films are relatively labor-intensive.
	The slightest damage to the barrier layer may negate the effectiveness of corrosion protection.
	Calculating the required number of desiccant units is not entirely simple and it is easy to overcalculate. However, too much protection is better than too little.
	Humidity indicators inside the package are not very reliable as they are only valid for certain temperature ranges. http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection. We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact. For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Classification of corrosion protection methods (Part 2) - Protective Coating Method

 Protective coating method


The protective coating method is a passive corrosion protection method. The protective coating isolates the metallic surfaces from the aggressive media, such as moisture, salts, acids etc..

The following corrosion protection agents are used:

	Solvent-based anticorrosion agents Very high quality protective films are obtained. Once the anticorrosion agent has been applied, the solvent must vaporize so that the necessary protective film is formed. Depending upon the nature of the solvent and film thickness, this drying process may take as long as several hours. The thicker the film, the longer the drying time. If the drying process is artificially accelerated, there may be problems with adhesion between the protective film and the metal surface. Since protective films are very thin and soft, attention must always be paid to the dropping point as there is a risk at elevated temperatures that the protective film will run off, especially from vertical surfaces. Since solvent-based corrosion protection agents are often highly flammable, they may only be used in closed systems for reasons of occupational safety.
	Water-based anticorrosion agents Water-based anticorrosion agents contain no solvents and thus do not require closed systems. Drying times are shorter than for solvent-based anticorrosion agents. Due to their elevated water content, water-based anticorrosion agents are highly temperature-dependent (risk of freezing or increased viscosity). The advantage of this method is that the protective film is readily removed, but the elevated water content, which may increase relative humidity in packaging areas, is disadvantageous.
	Corrosion-protective oils without solvent Corrosion-protective oils without solvent produce only poor quality protective films. Good quality protection is achieved by adding inhibitors. Since these corrosion-protective oils are frequently high quality lubricating oils, they are primarily used for providing corrosion protection in closed systems (engines etc.).
	Dipping waxes The protective layer is applied by dipping the item to be packaged into hot wax. Depending upon the type of wax, the temperature may have to be in excess of 100°C. Removal of the protective film is relatively simple as no solid bond is formed between the wax and metal surface. Since application of dipping waxes is relatively complex, its use is limited to a few isolated applications. http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection. We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact. For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com