Standards for intumescents

In North America, the only standard that adequately addresses serious environmental exposures and the longevity of intumescents is UL1709 (Standard for Safety for Rapid Rise Fire Tests of Protection Materials for Structural Steel) (2). Unfortunately, this standard only covers fire testing of structural steel columns against the hydrocarbon time-versus-temperature curve, which essentially restricts its use to fire protection of exterior steel structures in the oil and petrochemical industries (a minority of applications for intumescent products). In fact, testing against UL1709 can deter manufacturers of intumescents, and for good reasons, which are described later.

UL1709 contains a very tough set of tests, as it should, considering the rigorous applications for exterior hydrocarbon fire protection and the enormous risk potential for refineries and chemical plants. This test regime does nothing for interior products because no one has tested common interior passive fire-protection products to UL1709. One reason is that endothermic coatings such as intumescents generally contain large concentrations of epoxy coatings. Consequently, an unacceptably high amount of smoke is generated when the coating is first exposed to fire, and such a large fuel contribution violates building codes across North America.

Although other methods for testing intumescent materials are available, the best and most scientifically sound bench-scale testing is from DIBt. Under the DIBt approval guidelines, building materials are tested for interior or exterior applications. DIBt-approved firestopping, fireproofing, and gasketing products are available in North America; manufacturers include 3M (intumescent firestops) and Nullifire (a thin-film intumescent spray fireproofing product).

Committees from UL and UL of Canada (Toronto) are currently considering mandating the use of a modified version of the DIBt method for bench-scale testing of intumescent products used in firestopping applications. Neither U.S. nor Canadian manufacturers are enthusiastic about an environmental exposure mandate. However, manufacturer due diligence and providing useful and reliable data to the end user are the real issues because it is the costly environmental exposure testing as well as practical performance that causes some intumescents to fail.

Assessing the intumescent market

The largest market for intumescents is the industrial exterior spray fireproofing market. All manner of passive fire protection products compete for this market, including intumescents, other endothermic products, cementitious plasters, fibrous plasters, fibrous wraps, and cast concrete, as well as active fire protection products, such as the type of sprinkler systems used to protect liquefied petroleum gas (LPG) containers.

Let us look at a situation in which problems associated with rival products can be solved by intumescents: petrochemical plants, which contain process pipe bridges (structural steel racks for the purpose of holding up process piping), vessel skirts (round steel sheet structures, which support a vessel above), and spherical or cylindrical LPG containers.

There is a definite demarcation line within petrochemical facilities in terms of the importance given to (and willingness to part with funds to safeguard) pipe bridges and vessel skirts compared with LPG containers. Without vigilant enforcement measures, many above-ground LPG containers remain unprotected and thus subject to fire exposure in case of a flammable hydrocarbon spill. Facility owners who do pay to fireproof their LPG vessels are more likely, if aware of all technical aspects and expenses, to choose an intumescent or other endothermic product, rather than a cementitious or fibrous plaster. There are several reasons for this; one is longevity.

There is considerable variation in reinforcement factors and inherent flexibility between cementitious plaster products used for spray fireproofing (7). Plaster delamination and the corrosion of the steel mesh used to reinforce the plaster have in some cases caused the spray fireproofing to become dislodged and fall off. Common factors known in inorganic chemistry and in the concrete industry contribute to such events. Omitting the exterior waterproofing membrane or the priming layer permits weathering to occur, causing the cement-bound plaster to drop in pH value. This reduces the corrosion protection of the reinforcing mesh, which starts to rust, expand, and thus potentially damage the vulnerable plaster. This effect is especially pronounced in installations near the ocean, where the salt spray accelerates corrosion.

Experience has shown that because of faulty dew-point calculations and insufficient investment in quality materials and installation, fibrous plasters can become soaked with water and then freeze and delaminate. The owner of a petrochemical facility may find that the absolute lowest price is not the best tool for cost effectiveness in the long run.

Intumescent and endothermic products for this application circumvent the problems associated with the cementitious and fibrous plasters, for the most part. There have been cases of misapplication of intumescents (e.g., mixing incorrect proportions of ingredients) leading to the sliding off and total replacement of the product during the initial application. Intumescent coatings have also been known to delaminate; however, these are exceptional cases. One should use caution to choose a competent contractor. Intumescent and endothermic products are supplied in an epoxy paint base, so they are inherently corrosion-inhibiting. They are also much more likely to stretch and move with a sphere as it is being emptied and filled and undergoing weather changes. There can be no water or chloride penetration, and there is no cement to suffer from corrosive effects. Intumescent and endothermic products are significantly more expensive per square meter installed than fibrous and cementitious plasters. In light of the technical aspects, intumescent and endothermic products are worth the extra money, particularly when qualified to the environmental criteria of UL1709 or DIBt.

At present, there is no shortage of vendors for intumescent products, certified or not. If you are a manufacturer of passive fire protection products, it is certainly wise to be up-to-date on DIBt standards. I recommend qualifying your products with this method and obtaining a DIBt approval regardless of where your product may be sold, simply to prove due diligence as a manufacturer. If you are purchasing intumescents for your facility or for resale or you are specifying them for use in someone else’s facility, requesting a current DIBt approval and a system qualified to UL1709 is prudent. Beware that for each standard, not every exposure is mandatory.

Source: Achim Hering http://pubs.acs.org/subscribe/archive/ci/31/i05/html/05hering_new.html

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 

The Proof Is In The Fire

Intumescents are substances that swell when exposed to heat. The chemically bound water in intumescents absorbs heat, making them ideal materials for firestopping, fireproofing, and gasketing applications. Some intumescents, however, are susceptible to environmental influences such as humidity, which can reduce or negate their ability to function.

Although somewhat contrary to the true definition, intumescents are categorized as passive fire protection measures because they are used to compartmentalize fire (keep the fire in the location of origin). However, “passive” does not denote an absence of activity. During the intumescing process, physical activity (swelling) and chemical activity (loss of bound water and char formation) are both taking place.

A great deal of knowledge exists on the topic of intumescents, and there is no shortage of manufacturers and distributors for intumescents worldwide. The trouble is that many intumescent products do not work after they have been installed and exposed to common environmental conditions. An excellent source of information on the history and modeling efforts can be found on the Web

 Problems with intumescents

Some intumescents have very limited application because their beneficial properties (and in some circumstances, the applied intumescent itself) can disappear within days of installation. The culprits are various environmental influences, such as humidity (even normal indoor humidity), the UV component of sunlight, and heat generated by normal operations. Intumescents such as ordinary sodium silicates must be protected by epoxy or rubber coatings to ensure operability.

Most vendors of vulnerable intumescents are responsible and clearly state that their products require protection. But not all manufacturers are that responsible and forthcoming. For instance, using commercially available sodium silicates, any moderately skilled entrepreneur conceivably could design a product that will intumesce. The problem is the durability of the product. This is a significant factor, because some regions in Canada, for example, exhibit a wide variety of environmental conditions during a given year. There have been a number of product recalls for faulty firestops in Canada and the United States.

When a car manufacturer recalls a type of car, the owner drives it to the dealer, and the problem is corrected. But it is not quite that simple for passive fire protection products, which are concealed behind the finishes of occupied buildings. I doubt that you will find people digging holes into their drywall or masonry to replace faulty firestops or scraping deficient intumescent fireproofing off structural steel. Building owners are not always well informed of these fireproofing details because they have little to do with the original construction of the building. Thus, in creating the initial design of the formula, the manufacturer should consider the expected environmental exposures of the product and standardized testing and quality control regimes that will assure designers, contractors, and end users about the product’s reliability.

A product may intumesce in the laboratory, but will it perform the same way a week later in a desert, a jungle, or an arctic environment? At present, environmental exposure tests are not required in North America for most intumescent applications. Structural steel coatings are an exception and are discussed later.

Source: Achim Hering http://pubs.acs.org/subscribe/archive/ci/31/i05/html/05hering_new.html

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

Intumescent Coating vs. Cementitious Coating

Passive fire protection tends to fall into one of the following three categories: dense concrete, lightweight cementitious and intumescent coating. They are not all created equal. Recently, cementitious coatings have become less relevant to fireproofing a facility. As technology has progressed, intumescent coatings have superseded earlier methods of passive industrial fire protection as the most successful and cost-effective. Some of the reasons for that are discussed below.

Dense Concrete 
The potential of concrete as a fire-resistant material was recognized long ago. Many refinement facilities constructed prior to World War II made extensive use of dense concrete as a means of protecting against fires. The material is inexpensive and was known to withstand even extremely high temperatures. Problems quickly emerged, though. Concrete is heavy, which led to the need to over-specify structural steel. It also meant high labor costs, since forming concrete around steel is a laborious, multi-step process. It was also found that rapid cooling following a fire event leads to cracking in concrete and in some severe cases compromises the structural integrity of the material. This damage is sometimes difficult to detect and could become a danger to those working in the facility. Dense concrete as a means of fireproofing has largely been abandoned in favor of more recent techniques, which offer superior performance and fewer drawbacks.

Lightweight Cementitious 
Lightweight cementitious fireproofing retains the benefit of being based on inexpensive raw materials and without the problems associated with extreme weight. As its name suggests, the material is significantly lighter than dense concrete and so doesn’t require the over-specification of structural steel. But lightweight cementitious fireproofing retains the high costs of labor associated with dense concrete. It must be applied in several successive coats, again driving up labor costs. These products also share their predecessor’s tendency to crack. Perhaps the biggest liability with a cementitious coating, though, is the inevitable creation of space between the coating and the substrate. This space has a tendency to collect moisture, which in turn fosters corrosion of the substrate. In the long run, this unfortunate flaw can actually cause a lightweight cementitious coating to work against the integrity of the asset it was meant to protect.

Intumescent Coatings
Intumescent coatings work by charring and expanding in the presence of extreme heat. The increase in volume and subsequent decrease in density slows the heating of the substrate, increasing the time before the steel itself begins to melt. Intumescents typically swell to 25 times their original thickness when engulfed in flames. This expansion allows them to provide a barrier between the flames and the steel that is exponentially larger than a coating that does not swell. Adding thickness to an intumescent coating application increases the amount of swelling that will occur in the case of a fire incident. For example, if a 350 mil coating of a given intumescent has been determined to have a fire rating of 1.5 hours, 700 mils would theoretically be necessary to achieve a fire rating of 3 hours. In reality, though, added thickness is sometimes specified in certain areas such as curves and crevices, so something like a thickness of 750 mils may be required in order to achieve a 3-hour rating. When intumescent coatings come in single-component formulas, they are much simpler to apply than dense concrete and lightweight cementitious coatings and are therefore accompanied by far lower labor costs. Additionally, since they are applied directly to steel, no gap is created in which moisture can sit and incite corrosion. Intumescent coatings fight corrosion in much the same way as traditional protective coatings, the difference being their ability to swell and the much greater thicknesses at which they are initially applied. -

Mike Reed http://www.coatingsworld.com/contents/view_experts-opinion/2015-03-26/intumescent-coating-vs-cementitious-coating/#sthash.Grsl0ay6.dpuf

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

Types of Marine Corrosion (Part 2)

Stray Current Corrosion

We’ve discussed what galvanic corrosion can do, using just the electrical potential in metals. Imagine what happens if you add more electricity. That’s exactly the basis for stray current corrosion.

Stray current corrosion occurs when metal with an electrical current flowing into it is immersed in water that is grounded (such as in any lake, river, or ocean). The current can leave the metal and flow through the water to ground. This will cause rapid corrosion of the metal at the point where the current leaves. Stray direct current (or battery current) is particularly destructive. Stray current corrosion can cause rapid deterioration of the metal. If the metal in question happens to be an aluminum part like your drive unit, it can be destroyed in a matter of days.

Stray current corrosion is different from galvanic corrosion in that galvanic corrosion is caused by connections between dissimilar metals of your boat’s drive components, and utilizes the electrical potential of those dissimilar metals. Electrons flow from one dissimilar metal (the anode) to another dissimilar metal (the cathode). In stray current corrosion, electricity from an outside source flows into your boat’s metal components and out through the water for a ground.

For example, your boat may be sitting between a boat leaking DC current and the best ground for that current. Rather than the DC current moving through the water to ground, your boat could provide a path of lower resistance. The DC current could enter a throughhull fitting, travel through the bonding system, and leave via your drive to the ground. Remember that corrosion occurs at the locations where DC current leaves metal and enters water.

Stray current can come from an outside source either internal or external to your boat. Internal sources involve a short in your boat’s wiring system, such as a poorly insulated wire in the bilge, an electrical accessory that may be improperly wired, or a wire with a weak or broken insulation that is intermittently wet.

External sources are almost always related to shorepower connections. A boat with internal stray current problems can cause accelerated corrosion to other boats plugged into the same shorepower line if they provide better ground. The stray current would be transmitted to other boats through the common ground wire, but can and should be blocked by installing a galvanic isolator.

A much more subtle, but potentially more damaging cause of stray current corrosion can occur without any electrical problems. Supposed you cruise back to your marina after a weekend on the water, and plug into shorepower to recharge batteries using your automatic trickle charger. Then you go to work for the week. On Monday, a large steel hulled boat (with scratched and scraped paint) ties up next to your boat. This boat is also plugged into shorepower and goes visiting onshore for a few days. A battery has just been formed—the large steel hull and your small aluminum drive connected by the shorepower and ground wire. Depending on the proximity, relative sizes, and how long your neighbor is ashore, when you go out the next weekend you may find your drive highly deteriorated. This unfortunate scenario can also be prevented by the installation of a galvanic isolator.

There is greater danger for boats that connect to AC shorepower: destructive, low-voltage galvanic currents (DC) passing through the shorepower ground wire. Normally, AC is not a corrosion problem, but because the boat, pier, and wire are all connected, or due to a leakage, there can be direct current (DC) also present. This is potentially very damaging and requires additional protection.

Safety regulations require a three-wire cable for carrying shorepower aboard any boat, and that one of these leads grounds all electrical and propulsion equipment to shore. This safety procedure reduces the danger of shock, but also connects the underwater metal components on your boat with metal on neighboring boats using shorepower, steel piers, and metal objects on shore that extend into the water. This interconnecting of dissimilar metals allows destructive galvanic currents to flow between them. If these currents are allowed to continue, your drive unit will experience severe corrosion damage in a very short time—as little as a few days.

There is a common misconception that you can overprotect your drive by using too many zinc or sacrificial aluminum anodes. This is not true. The corrosion potential of any metal is a voltage that can be measured by a reference electrode. Such measurements in water commonly are made with a silver/silver chloride reference electrode. The corrosion potential of a sacrificial anode is a characteristic value for that metal, and it does matter if you have one piece of the metal or 100 pieces. The corrosion potential stays the same. Of course, 100 anodes would be expensive, heavy, and a considerable drag under water. Only by increasing the corrosion potential by using a different anode material (such as magnesium in seawater) can you overprotect your drive.

Crevice Corrosion

There is also a form of corrosion that affects many metals, particularly stainless steel, called crevice corrosion. A crevice may be formed under any of the following: deposits (such as silt or sand), plastic washers, fibrous gaskets, or tightly wrapped fishing line. It can also form where moisture can get in and not back out, forming a stagnant zone. Stainless steel is an iron-based alloy containing chrome and nickel. The quality that causes it to be stainless (nonrusting) is its formation of a thin, tightly adhering surface layer of chrome oxide. If this surface is deprived of oxygen, the oxide layer breaks down and the stainless steel will rust just like plain steel. In other words, stainless steel is only stainless when it has access to oxygen. In a crevice where there is moisture depleted of oxygen, stainless steel rusts. The simplest prevention for this condition is to seal out the moisture or clean off any deposits.

Antifouling Paint On Drives

Fouling is a major concern in many situations. Marine animals (barnacles, mussels, etc.) and vegetation can make life miserable for boaters. Antifouling paints are available, but some can affect corrosion protection or even accelerate corrosion.

In the past, tributyltin-(sometimes referred to as TBT or organotin) based antifouling paints controlled fouling and did not cause corrosion problems for aluminum drives. However, environmental concerns and legislation have restricted or prohibited the use of tributyltin paints. Presently, tributyltin-based paints must be applied by a state-liscensed repair shop. In the United States and Canada, tributyltin is prohibited for vessels less than 25 meters with an exemption for aluminum hulls, fittings, and drives. If TBT paint can be obtained, it is still recommended for drives.

Galvanic Isolators

Galvanic isolators are solid-state devices that are part of a series connected in line to the boat's green safety ground lead ahead of all grounding connections on the boat. This device functions as a filter, blocking the flow of destructive low voltage galvanic (DC) currents but still maintaining the integrity of the safety grounding circuit.

Inactive Sacrificial Anodes

If the underwater portion of the drive unit shows signs of corrosion, but the sacrificial anodes are not being consumed, the problem may be due to the following:

• The sacrificial anodes may not be making good electrical contact with the drive unit. Remove the anode, scrape the mounting surfaces on the part to be protected down to bare metal, and reinstall anodes.
• Zinc sacrificial anodes may have a protective coating of a very dense oxide film on their surface (which usually has a charcoal-gray appearance). This condition usually occurs in freshwater, but it can also happen in saltwater areas.

To confirm this condition, test for continuity between the anode and the drive using a multimeter set to "ohms" on the R x 1 scale. If the anode must be scraped with a knife in order to get a conductive reading, the anode is oxidized and should be replaced. Sanding the surface with coarse sandpaper provides a temporary solution, but the oxide will form again.

Some Cautions

Due to the location of the sacrificial trim tab, the drive unit must be kept in the "in" position when the boat is moored. If the drive unit is raised, the trim tab may be out of the water and, therefore, unable to act as a galvanic corrosion inhibitor. (Note from BoatUS editor: it is possible and preferable to place other additional sacrificial zincs on the bracket or elsewhere so that the motor can be raised.)

Do not paint anodes. Painting them will render them inoperative. The anodes will not provide corrosion protection when the boat is removed from the water, therefore the drive unit should be flushed with freshwater to remove saltwater and pollutants prior to storage. For example, dried salt deposits can react with moisture in the air or create a cell and corrode metal.

Do not attempt to use magnesium anodes in saltwater. They will provide overprotection. Over protection will result in a different electrochemical reaction that will create hydrogen on the metal surface of the drive, under the paint. The paint will blister and peel completely off the surface of the overprotected drive.

Corrosion Protection Testing and Troubleshooting

For diagnostic tests, a simple digital volt/ohm meter (multimeter) is necessary. An analog version may be used, but it must be a high-impedance model. Even the most inexpensive digital volt/ohm meter has high impedance.

One of the most helpful methods for determining if corrosion below the waterline is occurring is through the measurement of the hull potential. This is done by immersing a reference electrode, usually silver/silver chloride (a silver wire with a coating of silver chloride) into the water about six inches behind the drive. This electrode is connected to the positive terminal of a digital volt/ohm meter. The negative lead from the meter is attached to the battery ground. With the meter on a two-volt DC scale, the hull potential is displayed. When performing tests, be sure to make sure your battery is fully charged. Also, new boats will usually produce higher readings than normal. This is because the drive unit is being protected by a new finish and new sacrifical anodes. To obtain an accurate diagnosis, the test should be performed after the boat has been used at least one or two weeks. All boats should be moored for at least eight hours before performing the test. This is necessary in order to allow the cathode system and sacrificial anodes to polarize the water molecules in direct contact with the drive. Be careful not to rock the boat excessively while boarding to perform the test, as this will alter the reading. (Note from BoatUS editor: If you are not thoroughly familiar with safety procedures in dealing with electricity on the water, leave this to a qualified professional.)

The first signs of corrosion below the waterline are paint blistering, usually on sharp edges, and the formation of powdery white corrosion material on exposed aluminum surfaces. If the corrosion is allowed to continue, pitting of the aluminum will occur. The chart below may help you determine the cause of the corrosion and the corrective action needed to prevent its continuance.

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

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