Advanced shot blasting technology for processing of boiler internal surface before enameling

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1 Advanced shot blasting technology for processing of boiler internal surface before enameling Miklavčič M.*, Črtalič B., Trajkovski M. and Plavec J.,. *: FerroČrtalič d.o.o, Sela pri Dolenjskih Toplicah 47, SI-8350 Dolenjske Toplice, Slovenia, : FerroČrtalič d.o.o, Sela pri Dolenjskih Toplicah 47, SI-8350 Dolenjske Toplice, Slovenia : Slovenian NMR center, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia : EN - FIST Centre of Excellence, Trg Osvobodilne fronte 13, SI-1000 Ljubljana, Slovenia Introduction It is well established that enameling is the method of choice for preparation of endurable anticorrosion protection of boiler internal surface. The quality of the corrosion resistance essentially depends on the features of surface on which the enamel coating is applied. In particular, the absence of surface contaminants and appropriate surface roughness are prerequisites for optimal adherence of enamel to boilers internal steel walls. Processing of boiler internal surface before enameling usually comprises combination of various mechanical and chemical methodologies.[1] Typically, chemical pre-treatment of boilers includes the use alkaline cleaners and acid tanks to remove contaminants e.g. rust and hydrophobic substances from steel surface. Acid etching, in addition, promotes adherence of enamel to steel substrate. However, there are several concerns related to the use of chemical pre-treatment. For instance, cleaning agents and pickle liquor impose potential harm to workers and to environment. In addition, pickling results in modest surface roughness, which can compromise the firmness of enamelsteel bonding. As part of research activities aiming to characterize different adsorbents this study addresses the feasibility to use specialized shot blasting techniques for providing novel approaches for pre-treatment of steel prior enameling, which would enable avoidance of the drawbacks imposed by the use chemical pre-treatment. The thought that a single process could render boilers internal walls adequate for enameling seems unrealistic, but nevertheless very appealing. Shot blasting is a process in which abrasive grain i.e. shot blasting media is thrown on the surface at great velocities resulting in cleaning, e.g. removal of rust and scale accompanied with roughening of the workpiece s surface. Compressed air is used to blow away particles removed from the workpieces and crumbled small pieces of shot blasting media, which are collected in dust bins, while shot blasting media retaining the adequate features is recycled. Shot blasting is the final stage of boiler mechanical pre-treatment in industrial enameling line. Nowadays it often relies on the use of steel grit. This abrasive proved to be the optimal shot blasting media as it is highly recyclable, while providing optimal surface finish in terms of cleanliness and roughness, which enhances enamel adherence to steel. Regarding the specific requirements in terms of dimensions and types of boilers, processing capacities and available workspace, different kinds of specialized equipment for shot blasting of boilers interior had been developed. Notably, only carefully designed shot blasting equipment can render the required performance in terms of achieving the goal at rational costs shot blasting at the industrial level must be thorough and reliable and at the same time minimize wear off of shot blasting equipment (e.g. shot blasting media and nozzles), cost of labor as well as electric energy and compressed air consumption. Despite successful optimizations related to the aforementioned issues, a few concerns call for further exploitation of shot blasting performance. In particular, shot blasting suffers from intolerance to oils and greases comprised in metal working fluids (MWF). This limitation deserves special attention in the world of enameled products manufacturing. Namely, contamination with lubricant substances not only lowers recyclability of shot blasting media, but also compromises the quality of enamel coating. In the recent years the use of adsorbents emerged as a solution to the problems related to shot blasting of lubricated

2 workpieces.[2] The utilization of lubricant adsorbents, which are to be added to shot blasting media in the context of boilers interior pre-treatment prior enameling received hardly any attention in the past. Accordingly, the herein presented study aims to stimulate discussions and promote activities needed to further exploit the potential of shot blasting. Methods/Discussion Preparation of test plates and shot blasting media Lubricant, a typical metal working fluid, was deposited on cleaned steel plates at ca. 0.8 g/m 2. The plates were shot blasted with different media and handed over to enameling procedure, which comprised deposition of dry powder electrostatic enamel typically used for boiler enameling and firing at 830 C for 6 min. The labels and composition of different shot blasting media are given in Table 1. Shot blasting media additives comprised glass grit and different expanded perlites (EP). Shot blasted and enameled plates were designated with the same numbers as the shot blasting media used for their treatment. Table 1: Composition and labels of shot blasting media Shot blasting media label Abrasive a Additive b Lubricant [g] 1 steel grit glass grit 60 2 steel grit EP steel grit EP steel grit EP steel grit EP steel grit none 60 7 steel grit none 30 8 glass grit none none 9 steel grit none none a Each shot blasting media comprised 12.5 kg of abrasive. b The amount of additive in shot blasting media corresponded to 10 vol. % of abrasive. Shot blasting Modified pressure blasting machine PRESSO BLAST 3-33 chamber was used for shot blasting. During the mechanical pre-treatment the test plates were held fixed in position with their distance to nozzle being ca. 30 cm. The working pressure was 4.5 bars. Testing of individual shot blasting media was followed by thorough cleaning of the chamber and all of the components that were in direct contact with the media. Characterization of test plates Surface roughness of shot blasted test plates was analyzed by measuring R z obtained with the use of 'Mitutoyo Surftest SJ-210 Surface Roughness Gage. IR spectra in DRIFT technique were collected on shot blasted plates in order to compare the extent of lubricant retained on the plates after shot blasting. The spectra were recorded in the range between 4000 and 340 cm -1 at room temperature. Visual analysis was performed in order to assess the quality of enamel coating on plates that were pre-treated with the use of different shot blasting media and on lubricated plate which was not shot blasted prior enameling. Characterization of expanded perlites used as lubricant adsorbents comprised determination of specific surface area, size distribution and morphology of particles in the granular materials. Results Roughness of shot blasted plates was affected by each of the components used for preparation of shot blasting media, i.e. abrasive, additive and lubricant. Plates that were shot blasted with the use of steel grit exhibited between 2.4-times to 3.3-times greater roughness in comparison to the plates treated with glass grit as the only constituent of blasting media. Addition of lubricant or adsorbent in the form of granular media to steel grit resulted in (up to 28%) lower roughness of the workpiece (Table 2).

Detection of lubricant residing on testing plates was done with the use of DRIFT technique of IR spectroscopy, which is particularly suited for surface compound characterization (Figure 1).

3 Detection of lubricant residing on testing plates was done with the use of DRIFT technique of IR spectroscopy, which is particularly suited for surface compound characterization (Figure 1). Maxima at around 2925 cm -1 and 1001 cm -1 in the DRIFT spectra of testing plates that were shot blasted with the use of different media were assigned to stretching and bending of C-H and P-O bonds of the lubricate substances. Notably, the expanded perlites that were used as shot blasting media additive exhibited maxima in the range between 1250 cm -1 and 1330 cm -1 as well as around 1630 cm -1. The possibility that additives used for pre-treatment of plates 2-5 (entry numbers in Table 1) contributed to the absorbance at around 2925 cm -1 and 1001 cm -1 was therefore excluded. Concerning the extent of lubricant residing on testing plates as assessed from DRIFT spectra the testing plates could be arranged into four groups: (i) plates (2, 3, 4 and 5) treated with shot blasting media comprising steel grit, lubricant and additive different from glass grit; (ii) plates (1, 6 and 7) treated with shot blasting media comprising steel grit and lubricant or steel and glass grit and lubricant; (iii) plate (9) treated with shot blasting media comprising steel grit and (iv) plates (8). Table 2: Roughness of shot blasted testing plates Testing plate Mean value of R z [μm] Figure 1: DRIFTS normalized Kubelka-Munk spectra of steel plates shot blasted with the use of different (1-9) shot blasting media shown in the region a) between 2740 and 3740 cm -1 and b) between 500 and 2000 cm -1. Details on the composition of shot blasting media (1-9) are given in Table 1. Interestingly, the use of solely glass grit as the blasting media provided the most efficient treatment with regards to removal of the lubricant. On the other hand, addition of 10 vol. % of glass grit to lubricated steel grit only slightly enhanced removal of lubricate during shot blasting. The use of expanded perlites as additive not only failed to remove lubricant but even promoted its deposition from blasting media to surface of the testing plates. This indicated that the amount of the lubricant added to the shot blasting media reached or even exceeded the maximal adsorption capacity of expanded perlites. Additionally, in order for the degreasing additive to be effective, it should exhibit lower braking strength than the actual abrasive and the circulation of shot blasting media is required.[2] During the latter, thorough mixing is expected to result in the breakdown of additive particles and furthermore greater surface area exposure, which in general should contribute to efficiency of lubricant adsorption and removal of additive-

4 bound lubricant from the circulating shot blasting media. Notably, DRIFT spectra intensities, which hypothetically reflect efficiency of lubricant removal during shot blasting, indeed showed a good correlation with the specific surface area (SSA) of the expanded perlites used as additive in shot blasting media (Table 3) - the greater SSA of additive was correlated with reduced lubricant deposition to the surface of shot blasted plates. Table 3: Comparison between indicators of plate surface contamination degree and specific surface area of expanded perlites (EP1, EP2 and EP3) used as shot blasting media additive Testing plate a SSA [g/m 2 ] b at 2925 cm -1 c at 1001 cm -1 c 4 (EP3) (EP1) (EP2) a Expanded perlites used as shot blasting media additive are given in brackets. b Specific surface area of expanded perlites used as shot blasting media additive. c DRIFT spectra normalized Kubelka-Munk values. The types of abrasive and additive in blasting media was reflected in the amount of the lubricant deposited on the surface of plates prior enameling as well as on the quality of enamel. Enamel adherence to steel was analyzed in regions where enamel thickness was ca. 150 μm. Shot blasted plates (1-9) featured a very good adherence between enamel and steel (rank 1 on the scale of 1 to 5, where 1 corresponds to the maximal adherence), whereas very bad adherence (rank 5) was established for the plate that was lubricated but not shot blasted prior enameling. Enamel coatings on all of the plates, exhibited at least to some extent deficient quality due to the defects such as dimples, lumps and pinholes. Notably, defects such as burn offs in the regions where enamel thickness was between 100 and 140 μm were expected due to the intentional gradual increase of enamel application. It was previously noted that pinholes in enamel could result from too long blasting and too rough surface of the enameling substrate.[3] In our case, plate 8 exhibited low surface roughness and yet pinholes and small dimples in enamel were observed. Careful examination showed that the defects on plate 8 were present only at regions where enamel coating was thicker than 230 μm. Thus, the pinholes and small dimples on plate 8 at the region of extensive enamel application were assigned to unavailing expel of steel surface contaminants from molten enamel during firing. Apart from the mentioned, other enamel defects were not observed on the plate 8. On the other hand, the plates that were lubricated and pre-treated with the use of blasting media that included steel grit as well as the plate that was lubricated, but not shot blasted prior enameling all exhibited dark spots (Figure 2). Moreover, enamel of the later exhibited the greatest number of dark spots. Comparison of plates 6 and 9, both lubricated prior shot blasting, showed that the dark spots in enamel are bigger and more dispersed in the case when lubricated blasting media was used for pre-treatment. The fact that plate 8 did not exhibit dark spots overruled the possibility that the defects arose because of carbon boil due to high carbon content in the steel plates. Therefore, the cause of the dark spots was assigned to the MWF used to lubricate steel plates or/and to the shot blasting treatment, in particular the effect of shot blasting media. The lubricant deposited on plates prior blasting and added to shot blasting media represented only one of the origins that potentially led to contamination of plates surface prior enameling. Namely, inspection of fresh (not lubricated) steel grit used for testing exhibited brownish color indicating possible bad condition of the abrasive, which presumably included rusted particles. Additionally, there is a reasonable chance that even if the steel grit exhibited adequate quality, embedment of high carbon content particles from crumbled abrasive could be the reason for dark spots in enamel of the testing plates 1-9. This probability is supported by the fact that steel grit typically comprises between 0.08 and 1.2% carbon, which was also the case for the abrasive used in our testing. Notably, particle embedment during blasting, which compromises the quality of steel coatings, can be considerably reduced by adjustment of the angle at which media impacts the workpiece surface.[4] Regardless whether the lubricated plates were shot blasted or not, their enamel coating exhibited dimples, a defect commonly related to contamination of substrate surface. Interestingly, dimples were most evident for the plate 6, which also exhibited the biggest dark spots. Notably, according to DRIFT spectra the plate 6 exhibited neither the greatest neither the

5 lowest surface contamination prior enameling. Altogether, this indicates that dimples and pattern of dark spots in enamel could be related to dispersion of lubricant on the surface of testing plates, which furthermore depends on constituents of shot blasting media. Figure 2: Pictures of enameled plates captured at a) 60-times and b) 200-times magnification. Designations 1-9 correspond to the different shot blasting media used for shot blasting the testing plates prior enameling. Details on the composition of shot blasting media (1-9) are given in Table 1. * designates plate, which was lubricated but not shot blasted prior enameling. Conclusions The results obtained by surface metrology, spectroscopic and microscopic characterization showed that degreasing shot blasting media additives influence cleanliness, roughness and enamelability of shot blasted lubricated steel, however their use in a beneficial way is not straightforward. Without any details on the intricacy o frit manufacturing, enamel application and firing, this study affords a glimpse over the complex mutual contributions of individual parameters related to shot blasting that are crucial for quality of consecutive steel enameling. Our results point out the importance of addressing the adequacy of abrasive characteristics, amount and particularity of shot blasting media components (additives and eventual contaminants) and features of material during pre-treatment (presence of surface impurities after shot blasting, condition of abrasive and extent of particle embedment in the workpiece). Evidently, if shot blasting in the future is to supplant chemical cleaning of steel prior enameling, extensive research is inevitable. Acknowledgements We are grateful to Professor Jože Grdadolnik and Gregor Kapun for helping to conduct DRIFT spectroscopic and SEM analysis, respectively and to Mojca Opresnik for helping to determine specific surface area of the adsorbents. We would like to express our gratitude to companies Termika a. d. and Tectorius d.o.o for providing adsorbents used in the herein study and to company Emo Frite for their contribution regarding enameling. References [1] Andrews A. I., Pagliuca S., Faust W. D., Porcelain (Vitrous) Enamels and Industrial Enamelling Processes, 2011, Tipografia Commerciale, pp [2] EU patent No. EP A1, 2010 [3] Heid B., Heinz Frischat G., Hellmold G., Silicate Enamels for Stainless Steel, XXI International Enamellers Congress, pp , [4] Momber, A. W., Blasting Cleaning Technology, 2008, Springer, pp

NEW COATINGS FOR THE FUNCTIONALIZATION OF ENAMELLED SURFACES

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