January 2 - January 8, 2012
When performing maintenance painting of tank coatings, what are the pros and cons of overcoating an existing lining that appears to be in good condition?
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Christian Favennec of DCNS on
February 19, 2012:
To my opinion, it is not recommended to apply a new full coat over an old one in a tank, even and almost if it is assumed that the old one is in good condition.
The only result you will have is to generate new risks of premature failure:
- Risk of premature adhesion failure between the old coats and the new one.
- Risk of damaging the old coat by generating mechanical stress during surface preparation, with the stress generated by the new coat.
In a tank, the only need for application of a new full coat over an old coating assumed to be in good condition is for cosmetic reasons, generally on the ask of the customer and not for technical reasons. If you can assume that the old coating is in good condition, there is no need to apply a new coat.
The methods to define if the old coating is in good condition has been discussed in the precedent answers:
- Visual inspection for blistering, cracking, flaking etc. and repartition of these defects (localized or scattered).
- Adhesion pull-off tests
- Holiday detection
But generally on a old coat in a tank, you can detect the porosity or the missing by the visual inspection, and you can also damage the old coat if you use too high voltage. If you only have localized defects or less than 10-15% scattered, it's generally technically better and economically more interesting to only perform touch-up.
If you have more than 15% of scattered defective areas, I generally suggest completely blasting the old coat and reapplying a new one.
ishita Bhattacharya of Berger Paints India Ltd on
January 20, 2012:
Maintenance can be regarded as a cost-saving process. Periodic maintenence is very important, because if a tank coating is not properly maintained, then it can lead to premature failure of the tank. This can lead to discontinuity. The tank cannot be used while it is being repaired. Again, periodic maintenance is cheaper than large repairs, which will become a necessity in un-maintained tanks.
But keeping in mind the business and economic viewpoint, a judicious decision must always be taken regarding maintenance.The tank coating may appear in a good condition, but it has to be confirmed by testing: 1) DFT(ASTM D1186); 2) Adhesion X-cut (ASTM D3359); 3) the extent of corrosion (SSPC-Vis 2); 4) Pit depth (ASTM D4138). If the coating passes these tests, then it is actually in good condition and no maintenance is needed.
Tom Selby of Rodda Paint Corporation on
January 4, 2012:
The pro, of course, is the savings in time and money from not having to remove and dispose of the existing coatings. The con is whether or not there is enough adhesion to the substrate left and the soundness of all existing coatings on the surface. Prior to making a decision, adhesion tests should be done. If a sweep blast is done per SSPC-SP 7, then adhesion tests should be done after the sweep blast to check to see if the adhesion of the old coating system has been compromised in any way.
Simon Hope of Bilfinger Salamis on
January 9, 2012:
Vessel Coating Replacement Criteria: There are many thoughts regarding the timing for replacement of vessel linings and whether it should be a total or partial replacement.
Lining failures occur due to various mechanisms, the most common being the following: • coating thinning through abrasion • blister formation due to osmosis through the coating caused by surface contamination on the steel substrate during preparation. • solvent entrapment in the coating • adhesive failure of the coating to the steel substrate caused by surface contamination, inadequate profile, material applied outside its pot life or incorrect ambient conditions. • adhesive failure between coats, normally due to either contamination on the surface or material applied beyond the overcoating interval. • cohesive failure of the coating where a particular coat has torn itself apart, which can be caused by solvents, excessive film thickness or improper mixing • explosive decompression where molecules of a substance that at the operating pressure are liquid but are gaseous at ambient migrate into the coating during operation but in a rapid shut down situation blow sections of the coating apart. It may take several cycles to effect this result. • thermal stress cycling, due to differences in the linear thermal expansion characteristics between the substrate and the coating. Thermal cycling sets up a series of stresses which compromise the cohesive and adhesive strengths of the coating system. • mechanical damage from either internal or external sources • material aging, accelerated by elevated temperatures, especially with epoxies tending to become embrittled. • cathodic disbondment, where either an impressed current system or a local corrosion reaction cause the liberation of hydrogen from the substrate leading to the adhesive failure of the coating. • Coatings inadequately cured prior to being returned to service will exhibit many of the above properties either overall or in localised areas. • Exposure to excessive heat leads to breakdown of chemical bonds in the polymer and ultimately of the full coating system. • chemical attack of the lining due to the introduction of liquids into the vessel that destroy the coating integrity in either small or high concentrations.
The above list is not exhaustive but gives a feel for the problems associated with vessel lining performance. Because of the potential for the above failures to occur in a lining during service, it is a bit of a lottery estimating the life expectancy for a coating system. A vessel operating at ambient temperature and pressure with a non- aggressive liquid will have a greater lining life expectancy than a similar lining exposed to varying temperatures and pressures with an aggressive mixture if liquids passing through. Coating manufacturers on the whole will quote life expectancies for what they expect an ‘average’ exposure to involve.
When inspecting a vessel lining that has been in service for a period of time, any breakdown present needs to be carefully analysed to check whether it is a local failure with a driving force that is not throughout the vessel or whether what is encountered is the first stages of total lining failure.
This can be a hard call for the inspector as the decision on whether a total reline or a series of spot repairs could lead to a premature/ catastrophic failure of the lining when put back into service, especially when under pressure to get a vessel back online as fast as possible.
From an inspection point of view, a lot depends on the inspector to visually identify the defects present. The visual inspection should then be backed up by non-destructive tests to determine that the coating still meets specification, the most useful of these being to check the dry film thickness to look for wastage and coating thinning. DFTs need to be taken over the entire vessel, concentrating on the areas such as nozzles and edges where thinning of the coating is likely to be prevalent. Spark testing at 4Kv per mm of coating will show up any holidays along with other defects such as porosity and inclusions. Wet sponge holiday detection tends not to work on old coatings that have been immersed in an electrolytic medium or have a surface deposit of contamination as it gives false readings.
Destructive testing such as dolly pull off adhesion tests will give a good indication of any potential reduction in adhesion/cohesion but the test area requires to be repaired. Pull-off tests are also fairly lengthy to undertake as the adhesive for the dolly requires to be fully cured to achieve maximum strength; otherwise, the value of the tests is debatable. Inspection should identify if possible any problems that may occur during the repair procedure.
Spot repairing a vessel tends to be a short-term fix for a vessel lining as the repair itself can end up creating a weak spot. Preparation is invariably of a lower standard that that used to prepare a full vessel. Regularly, clients ask for power tool preparation of the affected areas, which does not give a long-term life expectancy for the lining. If blasting is used, it is essential to remove all corrosion salts and contamination from the affected area; chase back to a firm feathered edge; and at the same time, avoid collateral damage to the surrounding area that is not being treated. It is essential that all breakdowns are located and repaired though this invariably never happens.
Many of the above failures are not readily visible and can only be found by destructive testing during the early phases of the failure. Holiday detection will only work if the coating is clean and dry with no contamination on the surface, which will allow the test to track to earth giving bogus results. Spark holiday detection if done too often leads to failure of the coating and so is not really recommended for regular inspection.
Spot repairs give large amounts of ‘free’ edges where future breakdown may readily nucleate from. When blasting in a vessel to undertake spot repairs, there will inevitably be collateral damage from the blasting operation in the form of overblast and ricochet plus damage due to personnel and equipment moving around in the vessel. All of these can lead to coating weakness and potential failure. If a coating is reaching the end of its expected life, it is not advisable to spot repair except in extenuating circumstances. The failure rate will become unacceptable and damage to the vessel shell will occur. A new coating system should be used that will give the optimum life expectancy, consideration also to be given to the life of the vessel. Under most circumstances it is preferable to completely renew a coating system.
Changes in the operating parameters need to be taken into account as there may have been changes since the original specification was drawn up. It is worth looking at failures and using these to learn from to improve the specification.
Stripping the old lining off with UHP water jetting techniques has multiple benefits: • removal of the old coating • removal of corrosion scale • removal of contamination • removal of corrosion salts This leaves a substrate that can be rapidly dry blasted to the required standard with minimum risk of surface contamination; thus, maximum life expectancy for the coating should be achieved. Bearing the above comments in mind, applying this to the requirements for the Scott Separators, the following procedures and systems should be used: Initial vessel inspection: • 100% visual inspection after clean-out to identify through coating failures • dry film thickness check, all nozzles, edges and welds with at least 3 readings per m² to check for thinning/wear on the coating; minimum acceptable is 15% below specified thickness • 100% holiday detection to find any pinholing or porosity defects • Finally, if deemed necessary, apply dolly pull-off adhesion tests to check for reduced adhesion of delamination. • Report findings to client. Under normal circumstances, a failure rate in the order of 15 to 20% or greater would be deemed as uneconomical to spot repair, as the effort needed would be as great as that to recoat fully, the percentage being based on the percentage area of the vessel shell that will receive new coating, not the percentage area of visible failure or corrosion. What initially appears as a small defect of say nominally 10mmx10mm will probably end up as a repair of the order 400 to 500mm diameter if done correctly.
William slama of International Paint/Ceilcote Products on
January 5, 2012:
This answer is in regards to internal tank linings and particularly thick, reinforced thermosetting systems. First, one must focus on the functional challenges to the lining system, simplified here as permeation, direct chemical attack, and mechanical (i.e., abrasion, etc.). Next, the percentage of intact film area must make the "repair and overcoating" approach worthwhile, as opposed to removal and reapplication of the same or different lining system.
Basically, the existing lining system must be judged to have given and be able to provide good protection in the tank environment (primarily chemistry and temperature). In those cases, the existing lining damage could be due to application variables or localized challenges due to cold wall areas or abrasion. The intact lining areas should be assessed for remaining thickness and for through-adhesion with emphasis on evaluation of underfilm condition of the (steel) surface. If tensile adhesion strength through the film is acceptable (generally more than 1/3 of a new film), and underfilm corrosion is slight or none, overcoating is a good prospect. We have seen this in many high-build, reinforced linings that can be expected to have a protective life of 10-25 years. In some cases, localized areas have been compromised and/or the surface layer (top coat) has been severely degraded over many years, but the underlying lining is still strong and well bonded. The result has been that localized repair and reapplication of the top coat after abrasive blasting has resulted in 20-plus years of service without the much higher cost and down time of removal and relining.
jesse chasteen of schriener construction on
January 3, 2012:
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If a failure to less than 15% of the tank is discovered, address those areas only. Complete overcoating is money and effort wasted.
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