Corrosion Control During Layup

Garry Best Practises

Corrosion Control & Environment

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 Lay-up 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 companies provide recommendations on what needs to be considered as part of lay-up. Dehumidification & preservation are the two techniques most preferred. Classification societies recommend dehumidification as the preferred primary solution to eliminating corrosion & good paintwork as the preferred preservation solution where necessary.

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

Corrosion Control

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

Lay-up Solution Management

The following triangle figure 1 is an Engineering Hierarchy of Control where the most effective solution is eliminating the problem before it occurs. Therefore if humidity causes the corrosion problem, eliminating it with dehumidifiers removes the problem before it can occur and offer the most significant effectiveness. But, of course, this can only occur if the area can be enclosed; if not, another solution needs to be offered.

hierarchy of Corrosion Control

Figure 1 – Engineering Hierarchy of Control

Lay-up Environment

Figure 2 below shows the annualised minimum & maximum relative humidity from 1984 onwards taken at Changi airport in Singapore and is indicative of lay-up issues across tropical Asia. It offers an environmental snapshot of conditions that must be managed. The lay-up solution needs to consider these conditions and still offer an effective lay-up solution.

changi humidity graph

Figure 2 relative humidity minimum & maximum 1982 to 2015

 Corrosion Management

Corrosion control is essential so it can be eliminated effectively during lay-up. Sodium Chloride (NaCl) accounts for nearly 86% of all salts in the ocean. It is, therefore, the most significant contributor to corrosion. Salt deposition produces deliquescence (becoming liquid or having a tendency to become liquid) at 76% Relative Humidity (RH) for NaCl and 35% RH for MgCl. The corrosion current increases at the corresponding critical relative humidity area. See figure 3 below.

 

corrosion current

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

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

 

Again look at  Figure 2 above regarding the minimum & maximum relative humidity. You see that Asia offers the perfect corrosion cycle as it rises above 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.

representation of effects of corrosion

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

Solution for Internal Spaces

1. Dehumidification

It aims to eliminate moisture where an area can be enclosed. This strategy is suitable for accommodation spaces, machinery spaces & rooms, empty tanks & cofferdams. Classification societies recommend the following relative humidity readings.

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.

2. VCI Technology

It does not aim at the highest level of effectiveness called “Elimination” but at the secondary level of Engineering Control known as preservation. This method still has issues not resolved, such as humidity in electronics, the 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. Many gas detectors suffer deterioration even below this level. Concerning the above humidity chart in figure 2, many electronics start to fail with high humidity. If VCI is used 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.

3. 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

1. 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.

2. 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.

3. 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.

4. 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.