2. Case Study
Laser cleaning for Thermal Spray Application of Aluminium & Zinc Coatings
Date: 25.02.2015 – 27.02.2015
Site: SAPREF – Corner Roads 7 & 4
Persons present: Mike Webb – Integrity Projects
Laser cleaning Africa: Willem Breytenbach & Peter Nyoko
Southey contracting: group site workers, TSA Application crew
Equipment treated: Municipal water line 10” W18612
Surface description: Pipe on support areas, no coating in the immediate area, scale corrosion, surface corrosion, paint
failure.
Test requirements: To effectively remove surface corrosion and prepare the area to a standard that will support the
application of Thermal sprayed Aluminium and/or Zinc, to stop or reduce corrosion in these areas.
Background
Pipe to support interfaces have been a major area of concern for some years at SAPREF, in the past the systems that
have been tried have failed within months, Grit blasting being very abrasive is not considered as a safe or an effective
means of cleaning due to low remaining wall thickness readings (RWT) in these areas, water jetting and hand
preparation methods have not been successful due to not being able to clean in very small areas or pinch points, this has
meant that coatings have been applied on to a seemingly clean substrates but have actually entrapped the corrosion
which has intern accelerated the corrosion pocket.
The method of Laser cleaning is basically a pulsed beam of light that penetrates in to the smallest of areas and ablates
the surface removing rust and loose adhering contaminants leaving a small surface profile which we will coat with
Aluminium or Zinc leaving a *sacrificial coating that will stop or greatly reduce the corrosion in this area for many years.
*sacrificial coatings use a metal that is anodic to steel and corrodes preferentially.
3. Description of work
3 areas were identified for these trials, 2 at pipe to support interface and 1 on the top of the line for easy monitoring.
All the areas where pre cleaned with a water soluble degreaser and fresh water washed and left to dry. (To speed up the
process filtered dry clean air from the compressor could be used to force dry)
As the laser cleaning started it was evident that the light surface corrosion was removed easily and that the existing
coating around the perimeter was not damaged even after numerous passes this means the coating would continue to
give protection for some years, it is worth noting that after 3 – 4 passes with the laser all other passes did very little to
improve on the cleanliness visually, from this we based the cleaning on 4 passes to speed up the tests. We cut the
coating around the monitoring area to show the coating clearly before applying the zinc; this would not be done at the
pipe to support areas.
Figure 1: Test area on the left has been laser cleaned Figure 2: Test area cut before application of Zinc
Figure 3: Test area after coating with Zinc
4. The laser used was the 25 watt version and is close to being silent, there is very little dust compared to grit blasting and
no skips or extensive housekeeping afterwards, but the safety glasses come at a price of +/- R6000 a pair, recommend
only trained authorized personnel to be allowed in the 3M area to keep this cost to a minimum.
Figure 4: Pipe support area 1 Figure 5: Laser cleaning pipe support area 1
The weather was clear on the first day that we applied the Zinc coating with humidity between 65% - 74% and a
average temperature of 26°c, no preheating of the surface was done and the coating was applied as close to 250µm as
was possible due to limited access for the DFT gauge, the application is fast and easy. The Zinc overlaps the existing
paint coating with no problem and with no pre heating there is still no damage to the coating. (See figures 1 - 7)
Figure 6: Zinc application Figure 7 Close up of the application
5. After cleaning the 3rd
area on Friday, the Aluminium was applied, the weather was overcast with very slight rain in the
morning, coating was done in the afternoon, and it was 27°c with 78% humidity. Again no pre heating was done;
aluminium was applied to overlap the existing coating and to ensure the entire required surface was covered
Figure 8: 2
nd
pipe support area Figure 9 pipe support area 2
Figure 10: Aluminium application. Figure 11: The Aluminium fails over the existing coating.
6. Conclusions: The work done with the Laser went extremely well but a few points worth mentioning:
1. To increase productivity and where possible in the close proximity removal of thick loose material could be
removed with scrapers and a little sanding.
2. The operators that use the machine need a little coaching on what surface cleanliness is acceptable as
mentioned earlier - over cleaning achieves nothing and wastes time.
3. We used the 25 watt laser for the tests. The 100 watt laser will speed up the cleaning and the head size of the
laser remains the same so access will not be a problem.
4. The Coatings after 5 days including 2 days with heavy rain show no signs of rust staining.
Zinc is anodic to steel up to around 50C. At temperatures >50C it may become cathodic to steel and initiate
corrosion. At higher temperatures TSA is still the correct choice.
5. One concern is that the Aluminium coating cracks and disbonds from the painted coating this could lead to
premature failure if the aluminium is applied too thick.
You can enhance the adhesion by applying a thin coat of epoxy over the paint and then waiting for the epoxy
to gel slightly (tacky but not a liquid.) Next, put a quick pass of aluminum on top of the tacky epoxy, with the
hot aluminum particles embedding into the tacky epoxy. Wait about 30 minutes for the epoxy to harden and
then finish the aluminum thermal spraying. This method of aluminum over epoxy is called "Hot Flocking". It is
quite popular to do TSA over composite aircraft components this way
6. This is a trial; long term protection with this procedure is unknown.
I have seen some testing done in regards to zinc applied directly of rust on bridges and corrosion was
mitigated. I also did some testing myself, with zinc applied directly over rusted surfaces. Thus far, after 1 year
in a somewhat tropical Houston climate, my coating looks perfect! My PhD materials engineer friend also
believes that zinc applied over rusty carbon steel should mitigate corrosion.
Note that zinc thermal spray can and should be over coated with paint to extend its life. Also note that TSA
should never be painted except with a thin aluminum pigmented silicone sealer if cosmetic appearance is
important.
This is feedback courtesy of James Weber, Thermal coatings advisor to Shell
James K. Weber Consulting LLC
114 North Clinton Ave.
Bay Shore, NY 11706
X
Mike Webb
Integrity Projects
