Study on the protective performance of coating system for thermal spraying zinc on radar

In response to the harsh marine environment protection needs of radar, the protective performance of 9 types of coating systems for thermal spraying zinc was studied through condensation tests, salt spray tests, and temperature tests. The blistering phenomenon of the radar thermal spraying zinc matching protective coating system was analyzed. The research results showed that the blistering of the thermal spraying zinc matching protective coating system may be related to the pores of the sealing coating. Among the 9 coating systems, the comprehensive performance of B and D coating systems in terms of moisture resistance, salt spray resistance, and temperature stress resistance is significantly superior to other coatings. The radar using B and D coating systems has been successfully applied to various types of harsh marine environments, improving the radar’s protective performance.


Introduction
In recent years, the service environment of radar equipment has been expanding from inland and nearshore to offshore and ocean-going.However, compared to inland and nearshore environments, the corrosiveness of offshore and ocean environments has greatly increased, and the reliability and service life issues caused by corrosion put enormous pressure on the radar.In 3%~6NaCl solution, the corrosion potential of zinc is significantly lower than that of low carbon steel, so zinc has a significant cathodic protection effect on low carbon steel in marine environments, which plays an important role in the field of corrosion prevention of steel structures [1][2][3] .Thermal spraying of zinc is the use of thermal spraying technology to spray metal zinc onto the surface of steel substrates, which has a reliable cathodic protection effect and is an excellent long-term anti-corrosion coating.The thermal spraying layer has a porous structure, and its porosity can reach up to 15% depending on the spraying method and process parameters.Sealing the coating can reduce the infiltration of corrosive pollutants such as salt.Therefore, the composite coating of thermal sprayed zinc and sealer coat has a longer protection life than the simple spray coating [4,5] .The steel structure of radar mainly uses thermal sprayed zinc and organic coating as a long-term anti-corrosion solution, which not only provides long-term durability for the radar but also meets its decorative needs.In radar engineering practice, it has been found that the protective system of thermal spraying zinc matched with organic coatings sometimes experiences rapid deterioration such as coating blistering.This phenomenon is different from the atmospheric corrosion of zinc reported in the relevant literature, especially since its corrosion rate is significantly higher than that of ordinary atmospheric corrosion [6][7][8] , which should be related to the matching performance between thermal sprayed zinc and organic coating.Therefore, the matching performance between organic coating systems and thermal sprayed zinc coatings, especially the performance of sealer coats in organic coating systems, requires systematic optimization research.

Coating system design
The coating matching requirements for thermal spraying zinc surfaces in domestic and foreign standards such as ISO12944, NORSOK M501, GB/T30790, and GB/T28699 are generally composed of sealer coats and subsequent coats.Considering the dual requirements of radar products for protection and decoration, the coating system design was based on the scheme numbered A8.02 in the GB/T30790.5standard.The sealer coat was an epoxy coating, regardless of thickness, and the intermediate paint is an epoxy coating with a film thickness of 180 μm.The topcoat is a polyurethane or fluorinated polyurethane or polysiloxane coating, with a film thickness of 60 μm.The total thickness of the coating system was 240 μm.In terms of performance, it met the high durability (>15 years) requirements of the C5-M environment.All of the coatings were provided by domestic or foreign famous coating suppliers, and the coating system supporting plan is shown in Table 1.

Sample and preparation
The sample substrate was Q345 steel which commonly was used in radar steel structures, with a specification of 120 mm × 50 mm × 5 mm.We prepare thermal sprayed zinc coating according to the NACE No.12 standard, with sandblasting roughness Rz80 μm.Cleanliness Sa3 level, using arc spraying, has a thickness of 100% for hot spraying zinc μm~120 μm.
The coating system adopts ordinary air spraying, among which the sealing layer adopts fog spraying.The sealing layer is completed within 2 hours after hot spraying zinc spraying.The intermediate paint is painted on the second day after sealing, and the top paint is painted on the second day after spraying the intermediate paint.After spraying the top paint, it is cured at room temperature for 7 days before the edge of the sample is sealed.After curing at room temperature for 3 months, performance testing is conducted.

Testing
We conducted a pre-test evaluation according to GB/T30790.6,conducted a condensation test and neutral salt spray test according to the high durability requirements of the C5-M environment for thermally sprayed zinc coating in GB/T30790.6,and conducted a high-temperature storage test, low-temperature storage test, and temperature impact test according to the environmental conditions of radars.After each test was completed, we conducted a post-test evaluation according to GB/T30790.6.Test items, standards, and conditions are shown in Table 2.The adhesion evaluation before and after the test followed GB/T5210, the appearance evaluation followed ISO4628, and width evaluation was under GB/T30790.6.

Pre-test evaluation
According to GB/T30790.6 and GB/T5210, the adhesion evaluation before testing was conducted, and the results are shown in Table 3. From Table 3, it can be seen that the adhesion of the nine coating systems was larger than 5 MPa, and the failure mode was the detachment of the hot sprayed zinc layer from the steel substrate, indicating that the adhesion of the coating itself was higher than that of the hot sprayed zinc layer to the substrate, meeting the pre-test evaluation requirements of GB/T30790.6.

Condensation test
The condensation test applies to coating substrates, including porous and non-porous substrates.It is a continuous condensation performance test that may occur on the surface of the coating under harsh exposure conditions.It can reveal the phenomenon of coating damage including blistering and substrate damage.It is one of the widely used coating durability testing methods in standards such as GB30790 and ISO12944.According to GB/T 13893, a 720-hour condensation test was conducted using the continuous water cooling method.The evaluation results after the test are shown in Table 4.  4 shows that after 720 hours of condensation test, there was no significant change in the appearance rating of the 9 coating systems compared to before the test, and the coating systems were intact.However, from the adhesion data, compared to before the experiment, the adhesion of all 9 coating systems showed varying degrees of decrease.Among them, the adhesion of 4 coating systems A, G, H, and I decreased from above 5 MPa before the experiment to below 5 MPa, which no longer meets the requirement of adhesion ≥ 5 MPa after the coating system test in GB30790.However, although the adhesion of 5 coating systems B, C, D, E, and F decreased, it still exceeded the requirement of 5 MPa.The results of the condensation test indicate that the A, G, H, and I coating systems had poor moisture resistance and did not meet the high durability testing requirements under C5-M conditions in GB30790.The B, C, D, E, and F coating systems had good moisture resistance and met the high durability condensation test requirements under C5-M conditions in GB30790.

Salt spray test
Salt spray testing was conducted using two states: scribed specimens and unscribed specimens.The inscribed specimens were conducted appearance rating and adhesion testing, while the scribed specimens were conducted appearance rating and corrosion width rating.The scribed width of the sample was 1 mm, and it was necessary to scribe through the organic coating until the hot sprayed zinc metal layer was exposed.The results of rating and testing after the 1440 h salt spray test are shown in Tables 5 and 6.
Table 5 From Table 5, it can be seen that after 1440 hours of salt spray testing on the inscribed samples, there was no significant change in appearance and no coating damage such as blistering, rusting, or cracking level flaking occurred.However, the adhesion test after the salt spray test showed a significant decrease in the adhesion of coating systems A, H, and I compared to before the test, with adhesion data below 5 MPa.Table 6 shows that after the salt spray test, the scribed samples of coating systems A, C, E, F, G, H, and I showed obvious blistering phenomena in the coating.The scope of blistering was symmetrically distributed along both sides of the scribed line, and the inside of the coating blistering was a white zinc corrosion product.The corrosion width was calculated based on the distance between the blistering range and the scribed line, and the salt spray resistance of the coating system was judged based on the number and density of blistering.The order was: B=D>F>A>E>G>C>H>I.Based on the test results of both scribed and inscribed samples, coating systems B and D were significantly superior to other coating systems, meeting the 1440 h salt spray test requirements for high durability in C5-M environments in GB30790.
Comparing the inscribed and scribed samples, it can be seen that when the coating on the hot sprayed zinc was damaged, if the density of the coating was not enough, it was easy to cause corrosion media to spread internally along the damaged part of the coating, and the corrosion media accumulates in the pores of the zinc coating, causing zinc corrosion.The occluded battery effect formed by the pores and corrosion products intensifies zinc corrosion, and the loose and expanded zinc corrosion products cause paint film blistering.When the permeability of the sealing primer in the coating system is poor, it may cause the pores of the zinc spraying layer to not be completely sealed, and the residual air in the internal pores can also easily cause defects such as micropores in the sealing layer and subsequent coatings.The presence of these defects is equivalent to the damage caused to the coating by scribing in the test, resulting in blistering around the defects [9] .Unlike Ole Knudsen et al.'s description of blistering in thermal-sprayed aluminum coatings [10] , the blistering of thermal-sprayed zinc is due to defects in organic coatings, while thermal-sprayed aluminum is due to excessive coating thickness.Figure 1 shows the bubbles generated after the construction of a sealing primer with poor permeability.Figure 2 shows a comparison between the blistering of organic coatings on thermal sprayed zinc in radar and the scribed sample after 1440 hours of salt spray test.

Temperature test
Radar often undergoes complex environmental conditions during its service, including temperature changes and prolonged temperature stress conditions, which may lead to cracking and other damage to the fully cured coating system.The matching of the thermal-sprayed zinc layer with the substrate, organic coating with the thermal-sprayed zinc layer, etc. is a relatively strict assessment.Therefore, the coatings used in radar need to meet certain temperature test requirements.After continuous high-temperature storage, low-temperature storage, and temperature impact tests, the appearance rating and adhesion of the coating system are shown in Table 7. From the data in Table 7, it can be seen that after temperature testing, the appearance of the 9 coatings did not show significant changes, and there were no damage phenomena such as blistering, cracking, or flaking.However, the adhesion test showed a significant decrease in the adhesion of coating systems H and I compared to before the test, and the adhesion after the test was lower than 5 MPa, which did not meet the requirements for radar use.

Conclusion
When the sealing paint in the coating system cannot effectively seal the coating, the pores present in the hot sprayed zinc can easily cause micropore defects in the sealing layer and subsequent coatings, further leading to blistering of the coating system on the hot sprayed zinc.The results of the condensation test, salt spray test, and temperature test show that the comprehensive performances of coating systems B and D were better than other coatings, with excellent moisture resistance and salt spray resistance, meeting the requirements for high durability under C5-M conditions and meeting the temperature usage conditions of radar.Due to the better UV aging resistance of fluorinated polyurethane topcoat compared to aliphatic polyurethane topcoat, coating system B uses fluorinated polyurethane topcoat, which can be applied in areas with higher UV radiation such as high altitude and low latitude.Coating system D uses an aliphatic polyurethane topcoat, which can be applied in areas with lower UV radiation.Through the research in this article, the radar using coating system B and coating system D has successfully served in harsh marine environments, effectively improving the radar's protective performance.

Figure 1 .Figure 2 .
Figure 1.Bubbles generated during sealing of the hot sprayed zinc layer.

Table 7 .
Temperature Test Results.