In order to investigate the anti-corrosion effect of coated steel of steel bridge, Q345 steel plate specimens with three types of coatings, including zinc coating, aluminum coating and zinc-aluminum coating, are produced by the arc spraying technology. In the present study, chlorine corrosion tests are performed to investigate the influence law of different coating material and its thickness on the corrosion degree. Then the calculation results of two corrosion indicators are compared and analyzed. It is shown that the two corrosion indicators reflect the same corrosion law of three kinds of coating steel. The corrosion of all coated specimens is obviously severe in the early stage and gradually gentle in the later stage. It is also found that during the whole corrosion cycle, the corrosion rate of aluminum coating is smaller and change slower than the other two kinds of coatings, whereas the coating thickness of 200 μm of aluminum coating changes significantly. Therefore, aluminum coating is recommended as a priority, and the recommended coating thickness range of which is 100–150 μm.
1 Introduction
Owing to the advantages of high strength, light weight, good plasticity and toughness, and the continuous growth of steel production in recent years, steel structures have good prospects in the field of civil engineering. Now steel bridge plays a very important role in transportation power in China, but its corrosion in chloride salt environment has become one of the key problems affecting the durability of steel bridges. The corrosion of steel is more serious in the environment with high humidity and aggressive media, among which chloride ions is especially prominent. Therefore, protective measures such as rust removal, painting and coating must be taken to ensure the safety of the steel structures. Meanwhile, steel structures must also be regularly maintained, which spends a certain amount of maintenance costs. Experiment studies are carried out involved the corrosion behavior of steel in different marine areas and laboratory simulated marine environments. These studies mainly focus on the corrosion mechanisms of different types of steel as well as the components and properties of corrosion products. And whereas, the research on the corrosion rate of steel, especially coated steel, is yet relatively few. It is not only unhelpful for the accurate evaluation of the durability of steel bridges, but also for the reasonable selection of coating type and thickness. Therefore, it is very important to investigate the corrosion rate of coated steel adopted in steel bridges.
Corrosion rate is an important index to study the corrosion law of steel. Huang [1] and Zhu [2] point out that under the same corrosion cycles, the change rule of corrosion rate of different kinds of carbon steel is basically the same through analyzing the corrosion data. In addition, Schumacher’s empirical formula is applied to fit the corrosion data, and it is proposed that the slope of the linear part of the formula can be approximated as the long-term corrosion rate of carbon steel. However, this experimental formula does not take into account the influence of rust layer and other factors on corrosion rate. Yu [3], Pour-Ghaz [4, 5] and Pradhan [6] study on the corrosion rate of rebar in concrete in chloride environment by using self-made corrosion rate test equipment, and then the corrosion rate model of steel bar in corresponding environment is respectively established. The corrosion behavior of steel plated with galvanized, aluminum and zinc-aluminum alloy materials is studied in Refs. [7‐9]. It is shown that these three plating layers can restrict the corrosion rate and have a good corrosion protection effect for steel. The practice of Arc spraying is well established because of its high strength, excellent coating performance and high efficient energy use. Now, it is few report about the anti-corrosion behavior of arc thermal spraying coatings. So it is urgently needed to study.
Anzeige
In this paper, the corrosion rate of Q345 steel coated by arc thermal spraying is studied based on chloride corrosion test. Then the effect of different coating type and thickness on the corrosion rate is also discussed. Finally, the existing representative corrosion rate models are evaluated by comparison.
2 Experimental Test
By using a 3.5% sodium chloride solution, corrosion tests are carried out in the environment of constant temperature and humidity (temperature is 20 °C, humidity is 95%). The experimental cycle were 7, 15, 30, and 60 days. The shape of steel specimens is rectangular plate with a size of 50 × 25 × 2 (mm). Three coating types are designed in the experiments, which are zinc coating, aluminum coating and zinc-aluminum coating. Three coating thickness are made for each type, respectively, 100 μm, 150 μm, 200 μm.
The preparation of the coating is in accordance with GB/T9 793–1997 “thermal spraying zinc, aluminum and their alloys for metal and other inorganic coatings” and other relevant standards. After each corrosion cycle, the corresponding specimen is removed and the corrosion products on the surface of the specimen are cleaned. The floating rust on the surface of the specimen is scraped off with a blade. Then 1000 ml solution is made for pickling and rust removal of specimens, which mixed with 500 ml hydrochloric acid, 3.5 g hexamethylenetetramine and distilled water. Specimens are put into anhydrous alcohol to dehydrate and to blow dry. The specimens are taken out of the oven after 24 h, and the weight of the corroded specimens are measured.
3 Results and Discussion
3.1 Test Results
Two indicators of corrosion degree are calculated in this paper, which are corrosion rate and cumulative loss of thickness, respectively.
(1)
Corrosion rate
Anzeige
The weightlessness method is the most intuitive and reliable method to evaluate the corrosion resistance of metals, which can reflect the macroscopic corrosion rate of metals [10]. It is assumed that the specimen is uniformly corroded. The corrosion rate is calculated by
where \({\text{W}}\) is the corrosion rate. \({\text{W}}_{0}\) is the mass of the specimen before corrosion. \({\text{W}}_{{\text{t}}}\) is the mass of the specimen after corrosion. a, b and c are the length, width and height of the specimen, respectively.
(2)
Cumulative loss of thickness
The other indicator is cumulative loss of thickness [11]. The formula is given by
Where D is the cumulative loss of thickness of the specimen. Di is the loss of thickness of the corrosion cycle i. W is the corrosion rate. \({\text{t}}_{{\text{i}}}\) is the time of the corrosion cycle i. \(\uprho \) is the density of steel, the value is 7.8 g/cm3 in this paper.
The data recorded in the tests are introduced into these two equations, and the results of three coating types, which are zinc coating, aluminum coating and zinc-aluminum coating with different thicknesses, are obtained under each corrosion cycle. The calculation results are shown in Tables 1, 2 and 3.
Based on the obtained calculation results in Tables 1, 2 and 3, the curves of the corrosion degree indicators with time are drawn by Origin, as shown in Figs. 1 and 2, respectively.
Fig. 1
The curve of corrosion rate with time
Fig. 2
The curve of cumulative loss of thickness with time
The corrosion law of three coating types is basically the same in Fig. 1. In the early stage, the corrosion of various coated specimens is severe, and then decreases rapidly. When the corrosion continues for 30 days, the corrosion rate tend to be gentle. During the whole cycle, the corrosion rate curves of zinc coating specimens and zinc-aluminum coating specimens fluctuate greatly, while the corrosion rate of aluminum coating specimens is the smallest, continuing to decline.
It is not difficult to find that like the corrosion rate, the growth of corrosion loss of thickness is evident in the early stage and smooth in the late stage in Fig. 2. The cumulative loss of thickness of zinc coating is the largest, zinc-aluminum coating comes second, and aluminum coating is smallest. In other words, the corrosion resistance of aluminum coating is the best.
In addition, compared with other thickness, it is more severe corrosion in all aluminum coating specimens with 200 μm at the initial stage of corrosion, and the corrosion loss of thickness is bigger than 100 and 150 μm. Therefore, aluminum coating is recommended to be the optimal solution, and the coating thickness ranges from 100 to 150 μm .
4 Conclusions
This paper focuses on the influence of different coating categories and thickness on the corrosion resistance of Q345 steel. Corrosion tests are performed by using 3.5% sodium chloride solution, the results are present. Then the corrosion degree is determined by two indicators for comparison. Based on experimental studies associated with analytical studies, the key conclusions are as follows:
(1)
It can be shown that the corrosion evolution law of the three coating types is consistent according to the two indicators, the corrosion is severe in the early stage and gentle in the late stage.
(2)
The corrosion resistance of aluminum coating is better than that of Zinc coating and Zinc-aluminum coating. The corrosion rate and the cumulative loss of thickness of aluminum coating are smaller. It is recommended that aluminum coating is preferred, and the reasonable thickness range of coating is 100–150 μm.
Acknowledgements
This research was funded by [Natural Science Foundation of Hubei] grant number [2022CFB547], [Natural Science program of Xiaogan] grant number [XGKJ2022010097], [Innovation and Entrepreneurship Training Program for university students] grant number [202110528019, 202210340048].
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