Corrosion Control -During Layup

Marine Layup Services - VCI Preservation
Preservation of Equipment

High humidity is a problem whether you are looking to lay up in Africa or Asia. Look for the signs and understand and plan and prepare for lay-up.

Most companies are trying to balance cost efficiency with lay-up effectiveness with differing results. Part of the problem is the climate. Therefore it should be understood to see what the impact may be.

Consider the following:

Providing a good layup solution involves

 

  1. Corrosion control
  2. Eliminate seizure of equipment
  3. Understand failure modes
  4. Mitigate where possible
  5. Reduce the impact

Corrosion Control & Environment

Classification & insurance company’s provide recommendations on what needs to be considered as part of lay-up. Dehumidification & Preservation are the 2 techniques used with all classification societies recommending dehumidification as the preferred primary solution to eliminate the problem & good paintwork as the preferred preservation solution where necessary.

As such this article discusses item 1 above, corrosion control.

Corrosion Control

Understanding what layup is all about and how be cost efficient and layup effective and when & where to use each solution is paramount in getting the best value for money.

Layup Solution Management

The following triangle figure 1 is known as an Engineering Hierarchy of Control where the most effective solution is to eliminate the problem before it can occur. Therefore if humidity causes the corrosion problem then eliminate it with dehumidifiers will remove the problem before it can occur and offer the greatest effectiveness. Of course this can only occur if the area can be enclosed and if not another solution needs to be offered.

Hierachy of Control
Hierachy of Control

Layup Environment

Figure 2 below is the annualised minimum & maximum relative humidity from 1984 onwards taken at Changi airport in Singapore and is indicative to layup issues found across tropical Asia from layup in Batam to layup in Labuan, in the Bay of Brunei. As such it offers an environmental snapshot of conditions that have to be managed and the layup solution needs to take into account these conditions and still offer an effective layup solution.

Changi Humidity Graph
Changi Humidity Graph

Corrosion Management

Corrosion control is important to understand so that it can be eliminated effectively during lay-up. Sodium Chloride (NaCl) account for nearly 86% of all salts in the ocean and is therefore the largest contributor to corrosion. Salt deposition produces what is called deliquescence (becoming liquid, or having a tendency to become liquid) at 76% Relative Humidity (RH) for NaCl and 35% RH for MgCl. At the area of the corresponding critical relative humidity corrosion current increases, see figure 3 below.

In effect readings of 76% RH should be avoided along with cycling over then under this figure.

Again look at  Figure 2 above with reference to the minimum & maximum relative humidity and you will see that Asia offers the perfect corrosion cycle as it goes above the critical 76% RH on an almost daily event as the temperature rises & falls between day & night. Figure 4 below is another example whereby metal is sprayed with salt mist continuously and compared to intermittent salt water spray. It is another example of the corrosion current in action as indicated in figure 3 above.

Corrosion Current
Corrosion Current
Effects of Corrosion
Effects of Corrosion

Corrosion Management

Corrosion control is important to understand so that it can be eliminated effectively during lay-up. Sodium Chloride (NaCl) account for nearly 86% of all salts in the ocean and is therefore the largest contributor to corrosion. Salt deposition produces what is called deliquescence (becoming liquid, or having a tendency to become liquid) at 76% Relative Humidity (RH) for NaCl and 35% RH for MgCl. At the area of the corresponding critical relative humidity corrosion current increases, see figure 3 below.

In effect readings of 76% RH should be avoided along with cycling over then under this figure.

Corrosion Current
Effects of Corrosion

Again look at  Figure 2 above with reference to the minimum & maximum relative humidity and you will see that Asia offers the perfect corrosion cycle as it goes above the critical 76% RH on an almost daily event as the temperature rises & falls between day & night. Figure 4 below is another example whereby metal is sprayed with salt mist continuously and compared to intermittent salt water spray. It is another example of the corrosion current in action as indicated in figure 3 above.

Solution for Internal Spaces

Dehumidification

Is aimed at elimination of moisture where an area can be enclosed. This strategy is suitable to accommodation spaces, machinery spaces & rooms as well as empty tanks & cofferdams. Based upon this the following relative humidity readings are recommended by all classification societies.

Accommodation                        45%-55% RH

Machinery Spaces                     35%-45% RH

Other Spaces & Tanks              35%-45%RH

Note that higher readings in the accommodation are to stop wood items such as furniture splitting & delaminating of wall panels.

Marine Layup Services - Accommodation Dehumidifier Ducting
Accommodation Dehumidifier Ducting
Marine Layup Services - G1200 Pipelayer Layup-Up in Malaysia
G1200 Pipelayer Layup-Up in Malaysia
Marine Layup Services - Main Switchboard Preservation
Marine Layup Services Main Switchboard Dehumidifier Preservation

VCI Technology

VCI does not aim at the highest level of effectiveness called “Elimination” but at the secondary level of Engineering Control known as preservation. There are still issues not resolved by this method such as humidity in electronics, moisture level in motors or corrosion under insulation (CUI) for example. Most electronics have a maximum humidity of 95%RH and some even lower where saturation occurs at 90%RH or below, in particular sensors. A lot of gas detectors will suffer deterioration even below this level. With reference to the above humidity chart figure 2, a number of electronics will start to fail with high humidity if VCI is solely relied upon as a means of preservation then the failure modes of all the equipment have not been addressed and the reactivation cost is guaranteed to be higher.

Marine Layup Services Electrical Panel Preservation
Electrical Panel Preservation
Marine Layup Services - VCI Spraying
Rig VCI Spraying
Marine Layup Services - VCI Shrink wrap
VCI Shrink wrap

Do Nothing

Some companies resort to this method, and reactivation costs are the highest.

Conclusion

Dehumidification where possible is the preferred method of corrosion control and offers the best solution to electronic & electrical components over all other methods.

Solution for External Areas

Dehumidification

It is possible for specific equipment on deck. Still, it involves much work to enclose the space so humidity can be eliminated. In addition, as the highest effective method of corrosion control, it has limited applications. Still, it is worth at times considering specific equipment.

Paint Protection

It is the preferred method by classification societies. Any breakdowns in coatings are advised to be made good before/at lay-up to ensure a protective layer to salt spray/ mist. Paint is an ‘Engineering Control’ under the Hierarchy of Control and is considered the primary protection method in large open spaces.

VCI Technology

VCI is another method companies are looking at with certain benefits worth considering. As touch-ups are required, VCI should be cost analysed by making good upon paintwork before committing to its use. For the protection of open pressure vessels, tanks and rotating & sliding components, there can be many benefits gained with VCI technology. Still, its application needs to be considered for the right reasons. It is not a one-spray solution to a comprehensive lay-up preservation strategy. MLS does use VCI technology on an as-required basis.

Do Nothing

It can be legitimate if the vessel dry-docks upon lay-up. However, it should be noted that the repair cost can be high if the corrosion gets out of control. Therefore, if this method is adopted, a proper monitoring program should be implemented and intervention acted upon before the corrosion accelerates.

Hull

Paint Protection

Put paint protection is the preferred method. Sacrificial anodes are suitable for use where the hull condition has coating breakdown. Testing must be conducted regularly to ensure the proper protection is offered. Different steel grades require a different current reading to ensure adequate protection. MLS uses a special anode that has been specially designed for lay-up operations.

VCI Technology

Overall not suitable for large underwater areas but can be considered for closed sea chests and pressurised lines such as ballast tank lines

Conclusion

Whichever solution you choose for the internal spaces, external areas and hull, the cost versus effectiveness and what cost to do nothing need to be considered. A reactivation plan identifies the best solution to ensure that you, the client stay informed about the cost, benefit and time taken to reactivate. MLS can help with any of these analyses and contact us to discuss further.

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