Effect of Water-Based Zinc Flake Coating on Corrosion Performance of Spring Parts

Effect of Water-Based Zinc Lamellae Coating on Corrosion Performance of Spring Components

Abstract

This study examined the coating capability of different types of springs used in industry with water-based zinc lamellae coating and the effect of different coating layers on the corrosion performance of springs. In the blasting stage of the coating process, S20 (stainless steel shot) blasting media was used. Following the blasting operation, the springs underwent gap fill suitability testing, Hogaboom testing and dry adhesion testing in the coil spacing areas. In subsequent experimental studies, water-based zinc lamellae coating was applied to two different types of springs. At this stage, coating quality comparisons were made on 2-coat springs to determine the effect of coating on corrosion performance. For this purpose, a dry adhesion test was applied to the coated spring samples to determine whether the coating adhered to the surface, and salt spray testing was performed to measure corrosion resistance performance after coating. According to the results of the dry adhesion test applied after coating, partial coating fill was observed in the gap areas of tightly coiled springs with 2 coats. Additionally, no coating fill was detected on loosely coiled spring surfaces and in the gaps between coils. According to the results of the corrosion resistance test applied to the springs, it was determined that springs with a minimum coating weight of 24 g/m² exhibited high corrosion resistance. 1. Introduction Corrosion is the transformation of an element from a lower valence state to a higher valence state. Apart from gold and platinum, most metals are found in nature as oxide compounds. Two different types of corrosion can be mentioned: chemical and electrochemical (Campbell, 2008). General corrosion is an electrochemical corrosion type that occurs when metal on the entire surface undergoes corrosion to the same degree. It leaves deposits on the material and this easily provides us with information about the material's lifespan. It is generally observed in zinc, lead and aluminium. Uniformly distributed corrosion, the most common corrosion type, results in metal loss much higher than other types. Uniformly distributed corrosion can be easily controlled by applying different protection methods (Kakani S. and Kakani A., 2004). Corrosion protection methods are fundamentally divided into two categories: cathodic protection and protection through surface coatings. Cathodic protection requires polarization of corroded metals as a cathode and can be accomplished by pairing the metal to be protected with a more active metal or by applying external current. In the first method, the current required for protection is generated by the cell formed by the metal and anode pair. Anodes lose weight at certain rates as they dissolve during protection. In the second method, the metal and anode pair do not need to generate current. This is because the current required for protection is supplied from an external source (Kakani S. and Kakani A., 2004). Surface coating protection involves three types of coatings. These are organic, inorganic and metallic coatings (Kakani S. and Kakani A., 2004). In the 1980s and 1990s, this coating system began to be used as a solution to coating systems providing high levels of corrosion resistance in the automotive industry. Zinc lamellae coatings have been used in critical applications as an alternative to electroplating because the process does not generate hydrogen. The coating system is one of the most effective corrosion prevention measures that can extend the lifespan and operating capacity of steel structures. There are several different coating systems such as organic or metallic coatings, and among these, organic coatings such as zinc lamellae coatings are highly effective in preventing corrosion because of their self-healing properties against localized damage such as scratches and impacts that occur on the coated part (Sugimura et al., 2016). Today, particularly in the automotive sector, this coating method is widely used in many automotive components such as springs, nuts, connection brackets, door hinges, balance shafts, brake discs, clutch brake systems etc., which are exposed to impact, friction, chemical effects and corrosion (Sugimura et al., 2016). A spring is a machine element that exhibits large deformations under force and stores energy during this deformation. Springs have a wide range of applications. Springs perform many functions within a system such as applying force, controlling motion, damping, changing frequency, and measuring force and torque. Because of these properties, they are indispensable elements of many products produced. They are manufactured from many types of materials from metals to plastics. The shape and loading characteristics of springs vary depending on the application areas in which they are used. First and foremost, springs are elements used to store mechanical energy when they undergo deformation. For this reason, a good spring should be able to undergo significant deformation and return to equilibrium without undergoing any dimensional change. Design engineers serving in the engineering field often need to design springs during their professional careers. Although springs are often not given adequate importance in design processes, they are among the most important elements that need to be considered. If the spring used does not work as intended, satisfactory results will not be obtained from the product. For this reason, spring design must be carried out correctly before designing the product. This process will reduce the cost of the product, increase its efficiency and provide a long working life to the product. In this study, the coating capability of two different types of springs, tightly coiled and loosely coiled, with water-based zinc lamellae coating and the effect of coating on the corrosion performance of springs were examined.

2. Material and Method

The water-based zinc lamellae coating process carried out within the scope of the study included surface preparation and coating stages.

2.1. Surface Preparation Stage

Prior to zinc lamellae coating, surface preparation operations including degreasing and blasting were applied to the samples. For the degreasing operation, springs were passed through a degreasing machine with the aid of a helix. This machine has three compartments; in the first compartment, grease on the spring surface was cleaned using alkaline cleaning chemicals by spray method (Table 1). Subsequently, in the second compartment, pure water was sprayed to rinse away any chemical residue. Following the rinsing operation, the springs were dried in a drying oven at a temperature of 130°C to prevent moisture and prevent sand from settling in the gaps. With these operations, springs free of grease were prepared for the blasting process. The blasting process, a mechanical cleaning method, aims to remove all chemicals, metal scale and any rust that may have formed previously from the metal surface and prepare it for zinc lamellae coating. Springs degreased and dried were blasted for 12 minutes using two different sand types, S20 and S70, and subsequently dust residue was removed from the environment using air circulation and dust fans. [caption id="attachment_133705" align="aligncenter"] Table 1. Degreasing operation parameters[/caption]

2.2. Performance Tests Prior to Coating

Following the blasting stage, which is the surface preparation step prior to coating, the springs underwent gap fill suitability testing, Hogaboom testing and adhesion testing. First, the springs were placed in zirblast powder and gap fill suitability testing was performed by checking whether sand residue was present in the coil spacing areas. The Hogaboom test is a control method performed to determine whether the metal surface is suitable for lamellae coating after the blasting process. When preparing the Hogaboom solution, 30 g of copper sulphate solid is dissolved in 200 mL of pure water, 30 mL of concentrated sulphuric acid is added, and finally the total volume is made up to 1000 mL with pure water. During the test, springs are fully immersed in the Hogaboom solution in the beaker, left for 15 seconds and the part is removed from the solution. The adhesion test was performed by applying adhesion tape (3M No. 610 adhesion strength on steel 47 N/100 mm) to the surface of springs exiting the blasting machine without leaving air on the surface. Three passes of the same directional pressure were applied to the tape-coated surfaces using a 3M squeegee, and the tape was removed by applying force quickly from bottom to top. The presence of dust residue on the tape surface was examined according to the adhesion test scale (Figure 1).

2.3. Coating of Samples

The coating process is a chain of operations that begins with the coating stage and continues with pre-drying, curing and cooling stages. Within the scope of the experimental studies conducted, springs were divided into 2 groups to determine the effect of coating number on coating performance, and 15 springs were coated with two coats. Parts to be coated by the dipping method were loaded into a basket. Springs were immersed together with the basket into a tank containing coating chemicals. After the surface was completely coated, it was removed from the chemical environment and a centrifuge stage was performed to remove excess chemical from the spring surface. In the centrifuge process, an important stage in the coating operation, the centrifuge speed was kept constant at 320 rpm and the centrifuge time (30+30 seconds right and left) at 60 seconds. After centrifuging, the coated springs were dumped onto a conveyor belt and first kept in a pre-drying oven at 110°C for 10 minutes, then placed in a curing oven at 320°C for 45 minutes with the aid of the same conveyor belt to cure the coating chemical. Following curing, the coated springs were cooled to 37°C and the first coat of coating was thus completed. Parts in both groups were subjected to the same process once more by applying two coats of coating and the coating stage was completed. The working parameters for the coating stage are given in Table 2.

2.3. Performance Tests Following Coating

To determine the coating quality of two different types of springs coated twice, coating weights of the samples were first measured. Subsequently, these spring samples underwent dry adhesion testing and salt spray testing. The dry adhesion test applied after coating was performed in the same manner as the dry adhesion test applied after blasting. In the salt spray test, coated samples are placed in a salt spray chamber where a 5% sodium chloride solution by mass is applied as a spray. The temperature inside the chamber is 35°C and the humidity ratio is 95%. At the end of the test, white and red rust formation on the surface of samples exposed to this environment was observed. In a sample with good corrosion resistance, there should be no surface defects such as staining, deterioration, cracking, breaking, swelling, flaking, layer separation or paint adhesion failure after 480 and 720 hours of corrosion testing, and red rust should form in a maximum of 5% of the component surface.

3. Experimental Results and Discussion

3.1. Performance Test Results Prior to Coating In the experimental studies conducted, degreasing and blasting operations were applied in the same manner to springs in the first and second groups as the surface preparation stage prior to coating. Springs blasted with S70 and S20 sand were placed in zirblast powder following the blasting operation and checked for sand residue in the toothed areas. The accumulation control between the coils in both sand types is shown in Figure 2. [caption id="attachment_133953" align="aligncenter"] Figure 2. Zirblast Test Results (a) blasted tightly coiled springs (b) blasted loosely coiled springs[/caption] The test results of springs blasted with S70 and S20 sand subjected to Hogaboom testing are given in Figure 3. A homogeneous color formation was observed on the spring surface and it was determined that the blasted springs were suitable for coating. [caption id="attachment_133957" align="aligncenter"] Figure 3. Hogaboom Test Results (a) springs blasted with S70 sand (b) springs blasted with S20 sand[/caption] According to the dry adhesion test results of springs blasted with different sand types, it was observed that there was no contamination or sand dust residue on the spring surface (Figure 4). According to the performance test results prior to coating, no significant difference was observed when comparing parts blasted with S70 sand and S20 sand. However, since S20 is stainless steel shot, its use was found to be more suitable for filter maintenance of the blasting machine. [caption id="attachment_133960" align="aligncenter"] Figure 4. Results of residue control after blasting (a) tightly coiled spring (b) loosely coiled spring[/caption]

3.2. Performance Test Results Following Coating

Following zinc lamellae coating, the coating weights of the samples were first measured. On the surface of 2-coat components, tightly coiled springs showed coating weights of 24.6 g/m², 24.8 g/m² and 25.6 g/m² respectively, and loosely coiled springs with 2-coat coating showed coating weights of 26.8 g/m², 26.6 g/m² and 27.1 g/m² respectively. Following the dry adhesion test applied to 2-coat springs of different types, it was observed that the coating provided proper adhesion to the surface and did not separate from the surface with the tape test. However, coating fill was observed in the gaps between coils in tightly coiled springs (Figure 5). According to the salt spray test results applied to both types of springs to measure their corrosion resistance performance, red rust was not observed in 2-coat tightly coiled springs at 480 hours, and in 2-coat loosely coiled springs red rust was not observed at 720 hours (Table 3). [caption id="attachment_133963" align="aligncenter"] Figure 5. Residue control after coating (a) tightly coiled spring (b) loosely coiled spring (c) coating fill[/caption] [caption id="attachment_133964" align="aligncenter"] Table 3. Salt Spray Test Results Following Coating[/caption] [caption id="attachment_133966" align="aligncenter"] Figure 6. Spring Samples with Salt Spray Applied Following Coating (a) tightly coiled spring 480 hours (b) loosely coiled spring 720 hours[/caption]

4. Conclusions

Springs from which alkaline contamination was removed by degreasing were blasted with S20 and S70 sand with a diameter of 0.35 mm at 12 amperes intensity for 12 minutes, and surface controls were performed after blasting. When springs blasted with S70 and S20 sand types were compared, no significant differences were observed in the performance test results, and since S20 is stainless steel shot, the use of this sand type was found to be more suitable for filter maintenance of the blasting machine. According to the coating weights measured after the coating process, coating weights of 2-coat springs were determined to be minimum 24.6 g/m² and 26.6 g/m². In the corrosion test of the first group of tightly coiled springs, white rust was not observed after 192 hours and red rust was not observed after 480 hours. In the second group of loosely coiled springs, white rust was not observed after 240 hours and red rust was not observed at 720 hours. The studies conducted and tests applied demonstrate that water-based lamellae coating, which is applied by casting or bulk method and does not contain solvent, can be used in tightly and loosely coiled spring materials. 5. References • Campbell, F.C. (2008). Elements of Metallurgy and Engineering Alloys, p. 323-549, ASM International, Materials Park, Ohio. • FIAT 9.57513 (2017). ASTM B117 Corrosion Test Standard. • Kakani, S.L. and Kakani, A. (2004). Material Science, p. 382-394, New Age International, • Daryaganj, Delhi. • Sugimura, S. & Liao, J. (2016). Long-term corrosion protection of arc spray Zn-Al-Si coating • system in dilute chloride solutions and sulphate solutions. Surface & Coatings Technology, 302, • 398-409 • Vu, T. N. (2012). Selective dissolution from Zn-Al alloy coatings on steel. Material chemistry. • Universite Pierre et Marie Curie-Paris VI, (41) • NOF METAL COATINGS GROUP. GEOMET 321/500 Technical manual (EN). (2017)

Semih Ulubayrak Research & Development Process and Quality Engineer Uzman Kataforez Yüzey Kaplama San. ve Tic. A.Ş.