Corrosion Control During Layup

Gary Woolley Best Practises 0 Comments
ship rig corrosion control

Corrosion Control & Environment

Whether you are looking to layup in Batam or as far as layup in Labuan an understanding of the environmental factors that are in play in Asia is important to drive the correct layup solution that is most effective for your assets.

Most companies are trying to balance cost efficiency with layup effectiveness with differing results. Many layup operators have entered the market following the ongoing downturn and may not be giving the best solution for you but what is best for them. As such 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. or Reduce the impact

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.

effective layup control
Figure 1 – Engineering Hierarchy 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.

Figure 2 relative humidity minimum & maximum 1982 to 2015

 

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.

Figure 3 Corrosion current at 80% RH & 40% RH

 

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.

Figure 4 Effects of corrosion on continuous salt fog versus cyclic salt fog

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.

VCI Technology

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.

Do Nothing

Sad but true that a number of companies are resorting to this method and the cost will be the highest of all for reactivation. The further you are away from a major repair centre such as Singapore the higher the reactivation cost and longer the reactivation time.

 

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

Is possible for certain equipment on deck but involves a lot of work to enclose the space so humidity can be eliminated. As the highest effective method of corrosion control it has limited applications but worth at times considering for certain equipment.

Paint Protection

Is the preferred method by classification societies and any breakdowns in coatings is advised to be made good prior to/at layup to ensure a protective layer to salt spray/ mist. This is considered an ‘Engineering Control’ under the Hierarchy of Control and should be looked at as the primary method of protection in large open spaces.

VCI Technology

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

Do Nothing

It can be a valid strategy if the vessel is going to dry dock upon layup but should be noted that if the corrosion gets out of control then the repair cost can be large. As such if this method is adopted a proper monitoring program should be put in place and intervention acted upon before the corrosion accelerates.

Hull

Paint Protection

Simply put paint protection is the preferred method. Where the condition of the hull has coating breakdown sacrificial anodes are suitable for use and testing is to be conducted regularly to ensure the correct protection is offered. Different grades of steel used require a different current reading to ensure adequate protection. MLS uses a special anode that has been specially designed for layup 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 needs to be considered as well as what it will cost to do nothing. Either way a reactivation plan will help to identify the best  solution to ensure that you as the client stay informed as to the cost & benefit and how long it will take to reactivate. MLS is able to help with any of these analyses and can be contacted to discuss further. Hope this article helps.

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