26 POWDER COATING, April 2013

Abrasive blasting — not yourtypical pretreatment

The abrasive blasting pro cessprovides one of the best sur-faces to apply coating to,and when done correctly,will result in a long-lastingcorrosion resistant finish. Inthis article, I will provide a

basic understanding of the blastingprocess, provide test data that illus-trates the effectiveness of abrasiveblasting when used as the pretreat-ment component of the coating sys-tem, and address special considera-tions when using abrasive blasting.

If you have ever been involved in aconversation about pretreatment forpowder coating, you have invariablyheard words such as stages, immer-sion, bath, chemistry, pH, phosphate,water-break-free, and maybe evenflocculate (look it up — it is a realword). These are all words thatspeak to chemical pretreatment.However, I would suggest that abra-sive blasting and the terms used toconvey the varying degrees of sur-face cleanliness are also worthy ofunderstanding and use in our workas custom coaters.

According to Powder Coating: TheComplete Finisher’s Handbook pub-lished by the Powder Coating Insti-

Coater’s Corner

Chris McKinnon Aegis Industrial Finishing

tute (PCI), pretreatment is definedas “the preparation of a part prior tothe application of powder coating inorder to improve adhesion and corro-sion resistance.”1 While chemical pre-treatment achieves this by eitheretching the substrate or adding a thinfilm to promote adhesion and corro-sion resistance, abrasive blasting onlyremoves material from the top layerof the substrate. As a re sult, theexposed raw steel is highly reactivewith its environment be cause there isno chemical passivation to preventcorrosion. On the positive side, themetal is also free of oils, soils, millscale, or rust that was there prior toblasting. In light of the controlled sys-tem afforded by the chemical pre-treatment, how does a custom coaterprevent premature failure before thecoating is even applied once the prod-uct has been blasted?

Blasting basics. Abrasive blastinghas been a preferred surface prepara-tion for liquid applicators for years. Itis such a vital component of the engi-neered coating system, in fact, thatthere are three classification systemsthat we will likely see referenced inNorth America. These systems areprovided by the following three stan-dards bodies: NACE International,The Corrosion Society (www.nace.org);SSPC: The Society for ProtectiveCoatings (www.sspc.org); and theInternational Organization for Stan-

dardization (ISO) (www.iso.org).Table 1 provides a brief overview ofthe standard designation codes. It isworth noting that SSPC and NACEhave published these as joint stan-dards. In the case of the ISO stan-dards, shown in Table 2, there is arough approximation to the NACE/SSPC standards, but they should notbe considered equivalent.

These standards all outline the vary-ing degree of surface cleanliness thatis acceptable after the steel has beenblasted (there is also a standardavailable for the blasting of nonfer-rous metal and hot dip galvanizedmetal — SSPC-SP16). The require-ments that the end user has for theproduct will determine what degreeof surface cleanliness is requiredprior to coating, however it is myopinion that anything less than acommercial blast (SSPC-SP6) isinsufficient preparation for powdercoating. The problem posed by usingonly a brush blast (SSPC-SP7) isthat, according to the standard,tightly adherent mill scale, rust, andcoating may stay on the surface evenafter blasting is complete. Coatingover any of these three things willlikely result in premature coatingfailure unless the refinished productis stored in a controlled environmentthat eliminates corrosion (for exam-ple, an air-conditioned room that israrely used, such as a server room).

Figure 1Coating System A (top) and Coating System B (bottom) are shown prior toadministering the pull off adhesion test

Table 1NACE and SSPC surface preparationstandards

Table 2ISO surface preparation standards

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One aspect of abrasive blasting thatis not spelled out in the aforemen-tioned standards is the blast profile.For argument’s sake, before steel isblasted, the surface is flat with nopeaks or valleys in the surface of themetal. Once it is blasted, peaks andvalleys are created which effectivelyincrease the surface area of themetal. This increases the surfacearea to which the coating bonds, andas mentioned above, exposes cleanbare steel. The blast profile is the

measurement from the peak of thehighest point to the lowest point. Theblast profile can and should be mea-sured to ensure that the proper filmbuild is applied to fill in the valleysand cover the peaks. The depth ofthe blast profile will be affected bythe size and shape of the media andthe pressure at which it is forced outof the blast nozzle. A thorough dis-cussion of blast media and appropri-ate profiles is beyond the scope ofthis article. For further information,

I suggest speaking with your coatingsupplier to find out what they recom-mend for their coating or lining, yourblast media supplier, and your localabrasive blaster.

A tale of two tests

Going back to the defini-tion of pretreatment intro-duced at the beginning ofthis article, corrosion andadhesion are the two mainelements of the coating system thatshould be improved as a result. Thetwo tests referenced in this articlespeak to these elements directly.

Adhesion. Using ASTM D4541 TestMethod E2, our company conductedpull-off adhesion tests using thesame adhesive for each instance oftesting. The two coating systems arebroken out in Table 3. For the pur-pose of this article, the qualitativeresults will be the focus.

In the case of Coating System A, inall instances the failure occurredaround the 3,000-psi range (which isthe upper limit of the 20-millimeteradhesion dolly that was used). Onlyin the case of the top coat was theresome noticeable cohesion failure. Infurther testing with a stronger adhe-sive and higher-rated 14-millimeterdolly, we experienced the same typeof failure around the 5,000-psi range.I must stress — at no point wasthere adhesion failure between theblasted steel to the primer, nor adhe-sion failure between the epoxyprimer and the top coat.

In the case of Coating System B (sol-vent wipe over HRS), we did notexperience coating failure, but in -stead saw the mill scale separatefrom the mild steel plate at around2,100 psi. What is interesting here isthat even with a solvent wipe andtwo coat system, the coating adhe-sion is greater than the bond of themill scale to the base metal. The fail-ure in Coating System B was not thecoating, but instead the metal towhich the coating was applied.

In the book Corrosion Prevention byProtective Coatings, the author statesthat “[i]rrespective of most of itsother properties, the coating with

Table 3

Two coating systems are tested using the same adhesive

Figure 2Coating System A (top) and Coating System B (bottom) after the pull offadhesion test

Figure 3An excavator counterweight illustrates the difference between a SSPC-SP10 Near White blast (right) and an unblasted surface (left)

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28 POWDER COATING, April 2013

very strong adhesion to the surfacewill retain its integrity much longerthan one with less adhesion but otherstrong characteristics.”3 In both casesit would appear that adhesion over asolvent wiped surface and a blastedsurface is similar in performance.However, the corrosion test will revealsomething entirely different.

Corrosion. Using ASTM B1174, ourcompany partnered with a majorcoating supplier to test a variety ofcoating systems on hot rolled mild

steel plates. The duration of the testwas 4,032 hours (168 days). For thosewho are not familiar with the test, thestandard requires that the pieces beplaced in a cabinet for a predeter-mined amount of time and subjectedto a constant salt fog at 95°F and apH of 6.5 to 7.2. The test panels wereevaluated every 168 hours.

The sample plates that we submit-ted for testing were all coated withthe same exterior grade polyester topcoat, and only the surface preparation

and presence of the epoxy primerwere the differing factors in the coat-ing system used. Just before being putinto the salt spray chamber, each sam-ple plate was cut vertically down tothe substrate to introduce a startingpoint for corrosion.

This test does not correlate to anyparticular real-world environment,and instead should be used as a ref-erence point when evaluating thesuitability of a coating system for aparticular application and environ-ment. For this article, the results aresimply meant to demonstrate a qual-itative difference between abrasiveblasting as pretreatment and a sim-ple solvent wipe.

Table 4 describes and Figure 4 showstest specimens that were subjected tothe salt spray chamber. Sample platesA and B both presented as failuresonce the testing was complete. The dif-ference in time between them can onlybe attributed to the presence of theepoxy primer, which is an integral partof the coating system that was used inthe abrasive blasted sample plate C. InD, the sample plate was pulled after1,344 hours due to too much corrosionshowing up on the face of the panel;while it was not a fail due to coatingundercut, the panel exhibited toomuch corrosion to carry on. This is aresult of the coating thickness notbeing enough to cover the blast profile— and hence a very clear example ofwhy a single coat over a blasted sur-face is likely not a good idea.

Only one of the sample plates passedafter 4,032 hours of exposure to salt

Table 4

Results of specimens tested using ASTM B117

Figure 4Sample plates A (top left), B (top right), C (bottom left), and D (bottomright) are shown after testing using ASTM B117

Figure 5

A 5-ton truck frame is shown beingblasted in preparation for powdercoating

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spray hours — the plate that wasblasted to an SSPC-SP6 and coatedin an epoxy primer and a polyestertop coat. But the question of prevent-ing premature failure once the baresteel is exposed after blasting hasstill not been addressed.

Special considerations

In conclusion to this article, qualitycontrol must be addressed whenpreparing and handling pieces forblasting. Any oil or grease on the sur-face of the piece to be blasted must beremoved prior to starting. Failure todo so will contaminate the abrasivesused as well as deposit the oil on thepiece as it is being blasted and anyother pieces done in the same media.

Once the part has been blasted, itshould be coated within 8 hours.Depending on the relative humidity ofthe area that you are in, this windowmay change. However, even if corro-sion is not a concern, there are othercontaminants that can negativelyimpact the part prior to coating. Ide-ally the blasting would occur in thesame facility as the coating to mini-mize any chance of introducing causesof failure, but if you do not have thecapacity to do so it is worthwhile col-laborating with your blaster to makesure that your requirements for mate-rial handling and shipping are clearly

understood and followed. All it takesis one oily hand on a freshly blastedpart to ruin a coating after it is cured.And the cost of rework is always goingto outweigh any profit held in a job.Once the job is on the truck, even if itis in an enclosed truck, it should beshipped on warm dry days to avoidintroducing condensation on the sur-face of the part.

When used as part of a properlydesigned coating system, abrasiveblasting as pretreatment will provideyour customers with a lasting finishthat leverages the performance ofpowder coatings. PC

Endnotes1. Nicholas P. Liberto, ed., Powder Coating: The

Complete Finisher’s Handbook (Alexandria, Va.:Powder Coating Institute, Vol. 3, 2004), p. 477.

2. American Society for Testing and Materials(ASTM) International standard ASTM D4541— Standard Test Method for Pull-Off Strengthof Coatings Using Portable Adhesion Testers,see www.astm.org.

3. C.G. Munger and L.D. Vincent, CorrosionPrevention by Protective Coatings (Houston:NACE International, 1999), p. 50.

4. American Society for Testing and Materials(ASTM) International standard ASTM B117 —Standard Practice for Operating Salt Spray(Fog) Apparatus, see www.astm.org.

Editor’s noteFor further reading, see Powder Coat-ing magazine’s website at www.pcoat

ing.com. Click on Article Index andsearch by subject category. To submit aquestion, click on Problem Solving,then scroll to Coater’s Corner.

Chris McKinnon owns AegisIndustrial Finishing Ltd. with hisfather in Surrey, BC. As a thirdgeneration metal finisher (hisgrandfather started a platingcompany in 1948, and his fatherhas worked in powder coating for morethan 20 years), he is actively developingnew markets for powder coating and pro-viding those who will listen a greaterappreciation for powder coating. He has anMA in Business Leadership, holds hisNACE CIP Level 3 (#31504), and is amember of PCI’s Custom Coater SteeringCommittee. His company is focused on pro-viding powder coating and abrasive blast-ing to the local market and specializes inprocess-driven quality for pieces up to 38feet by 9.5 feet by 10 feet and 7,000 pounds.If you would like to contact Chris, he canbe reached at chris@aegisfinishing.com.

This column discusses problems encoun-tered by powder coaters during the dailyoperation of their powder coating lines.These are in-the-field experiences fromcoaters. Its intent is to provide practicalinformation to line personnel who coat allday to help them improve in their work. Ifyou would like to contribute to this col-umn, contact Alicia Tyznik, editor, at651/287-5620 or atyznik@cscpub.com.

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