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Article

Investigations on Changes in the Surface Geometrical Texture Caused by the Use of Corrosion Product Removers

by
Aleksandra Ochal
,
Daniel Grochała
*,
Rafał Grzejda
* and
Agnieszka Elżbieta Kochmańska
Faculty of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology in Szczecin, 19 Piastow Ave., 70-310 Szczecin, Poland
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(5), 539; https://doi.org/10.3390/coatings15050539
Submission received: 3 April 2025 / Revised: 25 April 2025 / Accepted: 29 April 2025 / Published: 30 April 2025
(This article belongs to the Special Issue Anti-corrosion Coatings of Metals and Alloys—New Perspectives)
Browse Figures
Versions Notes

Abstract

:
In addition to ensuring the functionality of objects used in the household, transport or industry at large, applied design focuses on aesthetic qualities related to the external form and condition of a surface. At the same time, there is a trend for plastic, rubber or aluminium objects made by moulding (both injection and casting) to look as if they were made of natural materials. This effect is ensured by properly designed and manufactured surface textures in the mould seats. However, the working surfaces of the moulds often corrode as a result of inadequate maintenance and storage. The aim of this study was to find out how popular agents on the market dedicated to corrosion product removal would change the surface geometrical texture. During the prepared experimental plan, it was also decided to investigate the properties in this respect of one of the popular drinks (i.e., cola) which is sometimes used in workshop practice as an alternative corrosion product removal agent. Based on the results of the study, conclusions were drawn about the short- and long-term effects of the corrosion product removal agents.

1. Introduction

Nowadays, in the production of machine parts, an important issue aside from the production itself is their delivery in a suitable condition to the customer [1,2,3]. While it is quite easy to protect products from damage related to changes in shape and external dimensions (e.g., by using appropriately designed and stiffened packaging), it becomes much more difficult to protect the surfaces of products from mechanical damage and corrosion [4,5,6].
The problem of corrosion arises in the tool management of plastics processing companies and affects tools such as injection moulds, dies or stamping dies [7]. Tools inadequately stored between changeover cycles can be subject to general or localised corrosion, depending on the materials they are made of and the environments they are in [8]. The corrosion of materials is still a significant problem in the economy, especially as plastics processing uses moulds and dies that are expensive to prepare and made from materials whose prices are affected by the cost of surface finishing.
Surface texturing operations are usually carried out with special machine tools and at a low surface productivity [9,10]. Most often, the finishing operations produce a texture that imitates a surface with geometrically complex patterns, thereby giving it hydrophobic or hydrophilic properties [11,12]. Sometimes, the intended effect of texturising the surface of a mould is to reproduce a surface resembling a material of natural origin [13] or other man-made synthetic surfaces [14]. Such textures are usually produced during labour-intensive and expensive finishing operations, e.g., by micro-milling [15,16], electro-erosion machining [17,18] or using laser surface texturing [19,20].
The surface geometrical texture (SGT) is related to the shapes and heights of the peaks, the depths of the valleys [21,22,23] and the geometrical arrangement of the irregularities created by machining, i.e., the so-called patterns [24,25]. In contrast, functional surface requirements are defined for parts designed to provide a high resistance to abrasive wear [26,27], where, in addition to the wide core of the material observable on the Abbott–Firestone curve, the low volume of the surface roughness peaks is important. Another particularly useful feature is the ability to retain protective coatings (including paint coatings) [28,29]. In this case, the surfaces should be isotropic, with evenly distributed irregularities in which the protective coating is anchored. In contrast, the surfaces of parts on which adhesive joints will be made [30] should exhibit low peaks with simultaneously deep valleys, the shapes of which will ensure good wettability.
Equally important in the manufacturing process of injection moulding dies and stamping dies, in addition to surface texturing, is the proper execution of auxiliary operations such as washing, transportation and maintenance [31,32]. Inadequately performed auxiliary operations can insufficiently protect the tools against corrosion, and can sometimes even be the source driving its development. The phenomenon of corrosion is understood to be the process of the environment acting on the material [33,34]. This phenomenon also refers to the action of moisture on the surfaces of objects made of metallic materials [35,36]. Corrosion is a major degradation process that leads to significant economic losses and safety risks in a variety of industries, including transportation, construction, energy and heritage conservation. According to global estimates, corrosion-related costs account for 3%–4% of each nation’s gross domestic product (GDP), highlighting the importance of effective corrosion prevention and management strategies [37].
The environments in which corrosion occurs and develops are water, soil and the atmosphere [38,39,40]. The moisture content and purity of the atmosphere are the main factors determining the corrosion of metals. In southern European countries, the relative humidity of the air is at least 50% for more than 250 days of the year, and in northern European countries, it is as high as 75% [41]. In addition, corrosion processes are also favoured by sulphur oxide [42], with which the atmosphere is constantly supplied. Salts (e.g., NaCl) are other factors that can exacerbate corrosion which occur frequently in industrial practices [43]. Salts can get into demineralised water, which is the basis for the preparation of process fluids and coolants. This phenomenon most often occurs as a result of the inadequate cleaning and flushing of technical installations in water treatment plants. Sometimes, the problem remains undetected, leading to the accelerated degradation of machinery and production equipment.
In the case of mould surfaces of dies and stamping dies, the vast majority of corrosion processes occur in a moist environment and are electrochemical in nature. The course of the corrosion process is very similar to the processes occurring in electrochemical cells—see Figure 1. A distinction is made between the anodic side and the cathodic side. On the anodic side, where the reaction process starts, chemical oxidation takes place. On the cathode side, on the other hand, a chemical reduction reaction occurs.
Corrosion products on the surface can be removed mechanically, by laser or chemically. The mechanical removal of corrosion products most often involves the use of abrasive tools or minimally invasive blasting [44,45]. When mechanically removing corrosion products, it is most important to act directly on the corrosion foci by peeling them off of the surface of the object. It is also possible to apply laser surface treatment using the irradiation of the steel surface with a moving ring to remove corrosion products and contaminants [46,47]. The chemical removal of corrosion products is also a very popular method [48,49]. This involves the use of solvents and dedicated corrosion product removers. The advantage of chemical corrosion product removers is the use of corrosion inhibitors that, once the corrosion products have been removed, delay the further development of corrosion and the formation of new corrosion foci.
A survey of the literature on the subject reveals that the effect of the SGT on the corrosion mechanism or corrosion resistance is often considered. At the same time, as is evident from a review by Dwivedi et al. [50], many issues in this area remain unexplored. A summary of the research by Hilbert et al. [51] shows that the SGT is an important parameter for surface corrosion resistance. Also, Li and Li [52] found that the SGT can significantly contribute to corrosion, and thus to corrosion wear. Their experimental study showed that the corrosion rate increased with the increasing surface roughness of samples made of copper. Similar results were reached by Seo et al. [53] for 690TT alloy surfaces; by Jawwad and Mohamed [54] for stainless steel pipe surfaces; and by Šolić et al. [55] and Bai et al. [56] for flat steel samples. Pu et al. [57] proved that the process of severe plasticity burnishing of the surface significantly improved the corrosion resistance of an AZ31B Mg alloy in NaCl solution.
In contrast, the effects of chemical agents on the SGT have been studied less frequently. Parapurath et al. [58] reported that the surface roughness of low-carbon steel samples increased exponentially during etching in HCl solution. An increase in roughness was also observed for 316L stainless steel samples chemically etched in H2SO4, HClO4, NaOH and Na2CO3 [59]. On the other hand, it should also be noted that exposure to chemical agents on enamel surfaces does not affect their roughness [60], while exposure to chemical agents on fused quartz surfaces may even slightly reduce their roughness [61]. Sun et al. [62] analysed the effects of corrosion product removers on the surface morphological structure of carbon steel samples. Their tests revealed that the surface of the low-carbon steel treated with a neutral corrosion product remover was relatively smoother, and the rust was able to react with the corrosion product remover to form Fe-O-P and Fe-O-C chelate layers. In addition, the rust-removing samples showed a better corrosion resistance than the rusty samples.
In workshop practice, commonly available carbonated cola drinks are used to remove corrosion products instead of dedicated chemical agents. This is a practice that relies on established habits and limited time when corrosion product removal activities need to start immediately. Unfortunately, this involves the possibility of damaging the SGT, which has already been strained by the development of corrosion processes. The effects of some chemical agents, including cola drinks, can be so strong that the textures and shapes of the surface irregularities are altered along with the corrosion products. While chemical manufacturers include the technical conditions for the removal of corrosion products in their agent specifications, there are no reliable data on whether and under which conditions the removal of corrosion products with cola drinks can actually be carried out.
As the issue of SGT changes caused by the use of corrosion product removers has not been fully recognised, this study examines this phenomenon with reference to popular agents on the market dedicated to corrosion product removal. The possibility of using cola for this purpose as an alternative corrosion product removal agent is also investigated. The aim of our work was to investigate whether corrosion products can be removed from the surface of parts that have been previously textured without disturbing the textures (i.e., without changing the SGT state). The methodology developed involves tracking the changes in the roughness height and surface volume parameters.

2. Research Method

Prior to the development of the experimental plan, preliminary tests were carried out on the surfaces of the test samples. During the preliminary tests, the sizes of the corrosion products were determined in relation to the magnitude of the salt solution percentage concentration and the development times of the corrosion products. A surface finish milling programme was also developed at this stage. Industrial chemicals for the removal of corrosion products were reviewed. At the end of the preliminary tests for the two selected chemicals and the cola drink, the timeframe within which the corrosion product removal process would be carried out was determined.
Six samples (three identical pairs) measuring 20 mm × 20 mm × 20 mm were prepared from a 42CrMo4 steel with relatively low corrosion resistance. The chemical composition of the steel used, apart from Fe, was 0.38%–0.45% C, 0.60%–0.90% Mn, 0.90%–1.20% Cr, 0.15%–0.30% Mo and 0.10–0.25% Si. The 42CrMo4 steel was selected for the study because it is widely used in the manufacture of machine parts and mechanical components due to its high strength, ductility and good hardenability. Its widespread industrial applications makes it a relevant and representative material for surface degradation and restoration studies. This material is often used for machine parts and tools such as injection moulds, dies and stamping dies.
The samples were tempered to a hardness of 35 ± 2 HRC prior to surface milling. The surface milling was carried out using a Mithsubisi SRFT10 finger milling cutter fitted with a VP15TF circular insert with a radius of 5 mm (Mithsubisi Corporation, Tokyo, Japan). Machining took place on a DMG DMU-60 MonoBLOCK milling centre (DMG MORI, Bielefeld, Germany) with the tool positioned at 15° to the rotational axis of the machine spindle. The surface milling was conducted under conditions typical of surface finishing according to the technological parameters recommended by the tool manufacturer. The machining resulted in a surface with a single dominant direction and repeatable intervals of surface roughness.
The experimental plan was to prepare samples characterised by the texture resulting from the applied spacing milling feed fs of 0.5 mm. The remaining technological parameters of the machining were also the same in all cases, i.e., the cutting speed vc was 100 m/min and the feed per blade fz was equal to 0.1 mm (see Figure 2).
In this way, three pairs of samples were obtained, on the surfaces of which the effectiveness of the following corrosion product removers was tested: OPN-Rust Remover (R1) (OPN-CHEMIE GmbH, Neunkirchen, Germany), Epoxy Brunox (R2) (Brunox AG, Neuhaus, Switzerland) and a cola drink (R3), which contained approximately 10 g of sugar and 50 g of CO2 per 100 g of the drink. There were A and B samples in each pair. However, this article only shows the results for the A series of samples from each pair.
The selection of the agents was based on their representativeness of the different approaches to corrosion product removal, namely as follows:
  • OPN-Rust Remover (R1) is a professional aerosol for cleaning corrosion products based on petroleum-derived hydrocarbons, including C10–C13 alkanes and isoalkanes, as well as 2-butoxyethanol and light naphthenic distillates. These components dissolve corrosion products through solvating and penetrating actions, while the formulation is designed to act gently on the base metal. Importantly, this agent does not contain passivating additives and relies on mechanical removal post-application. OPN-Rust Remover (R1) provides gentle cleaning without structural damage, but requires follow-up protection to prevent re-corrosion.
  • Epoxy Brunox (R2) is a commercial-grade corrosion product converter and epoxy-based primer, combining organic acids (e.g., formic acid) that dissolve iron oxides and epoxy resin components, which then form a protective polymer layer. This dual-function product both removes corrosion products and prepares the surface for coating, which contributes to its superior and more permanent surface stabilisation. Epoxy Brunox (R2) is chemically active, both removing and inhibiting future corrosion through a stable epoxy barrier.
  • The cola drink (R3) is a consumer-grade agent containing phosphoric acid and carbon dioxide, with a pH of ~2.5. Its weak organic acid content enables the short-term dissolution of corrosion products. However, due to the lack of protective ingredients and the presence of sugars, it may lead to residue formation that traps moisture, resulting in the renewed development of corrosion sites over time.
A comparison of the chemical composition of the OPN-Rust Remover (R1) and Epoxy Brunox (R2) agents is included in Table 1.
Four series of measurements of SGT were planned, during which the condition of the surface was assessed after milling, and then after corrosion had been induced. The surfaces of the samples were cleaned for 10 s each time with methyl alcohol and blown off with compressed air prior to measurement, so that there were no mechanical interactions. The samples were then allowed to dry for about 30 min at ambient temperatures. While the corrosion products were being removed, the samples were flooded with the chemical to cover and left in a sealed container for a specified time.
Two exposure lengths for the corrosion product remover were selected. The first was 2 h (Step 5—Figure 3). After cleaning with alcohol and blowing off with compressed air, further measurements of the SGT were taken, and once completed, the samples were put back into the sealed containers for another cycle of corrosion product removal. In the second case, the chemical dissolution of the corrosion products took 40 h (Step 7—Figure 3).
The lengths taken in the study are justified by industrial practice. A duration of exposure equal to 2 h is recommended by chemical manufacturers. On the other hand, a duration equal to 40 h is an extreme duration of exposure justified by economic conditions. If it is not possible to remove the corrosion products from the surface of the object by chemical means within this time, other mechanical cleaning methods are used. The test plan developed consisted of a total of eight steps, and the testing of the individual samples was staggered so that the measurements of their SGTs did not interfere with the lengths of time of the chemical surface cleaning of the other samples.
Corrosion products at several locations on the surfaces of the samples were produced under laboratory conditions at a temperature of approximately 21 °C and at a humidity level of 50%. Locally, drops of a 3% salt solution were pipetted onto the milled surfaces of the samples. Due to the evaporation of the drops of salt solution, the application was repeated three times at one-hour intervals. The samples were then left for 20 h. This was followed by photographic documentation and measurement of the SGTs of the corrosion products formed.
The SGT measurements were carried out using an AltiSurf A520 multi-sensor surface topography machine (Altimet, Thonon-les-Bains, France) armed with a CL1 chromatic confocal sensor [65], with an operating range of up to 130 µm and a vertical resolution of 8 nm. The measurements were taken in 4.0 mm × 4.0 mm fields. Experimentally, the scanning resolution was set at 0.47 µm along the X-axis and 5 µm along the Y-axis, resulting in almost 8.500 points in 801 lines. The measurement of approximately 6.8 million points for each surface took an average of 6 h. The analysis of the collected data and the processing of the SGTs were carried out in accordance with ISO 25178-2 [66,67] using MCubeMAP 8.1 software (Mitutoyo, Kanagawa, Japan). Each time, a surface topography analysis methodology was applied to the recorded point cloud of the measured surface, which included setting a threshold value to remove erroneously collected surface points (the deleted points were set as unmeasured values). The surface was then levelled, with the mean plane approximated by the least squares method.
The following roughness and volumetric indices were chosen to describe the SGT [68,69,70]:
  • Sa—arithmetic mean surface height (μm);
  • Spk—reduced peak height (μm);
  • Sk—core roughness depth (μm);
  • Svk—reduced valley depth (μm);
  • Vmp—surface material peak volume (mm3/mm2);
  • Vmc—surface material core volume (mm3/mm2);
  • Vvc—surface void core volume (mm3/mm2);
  • Vvv—surface void valley volume (mm3/mm2).
All measurements were guided by so-called good metrology practices. At the preliminary test stage (i.e., before the start of the main part of the study), the developed SGT measurement methodology was validated. The measurement system, together with the developed measurement methodology, was validated on the surface of the sample after milling, which was used with the R1 rust remover in a further step. For this purpose, five scans were taken of the same section of the test area. Five sets of values of the selected SGT indices were then determined, from which the standard deviation σ S x   was calculated. The measurement uncertainty U S x was calculated with a defined value of the measurement uncertainty expansion factor k = 2.0 (covering 95% of cases around the mean value [27]) from this formula:
U S x = σ S x · k
The measurement uncertainty of the measurement system used was 0.29% for the Sa, 3.34% for the Spk, 1.17% for the Sk, 5.74% for the Svk, 3.2% for the Vmp, 3.93 for the Vmc, 2.61% for the Vvc and 3.33% for the Vvv. In the subsequent series of tests, the values of the surface roughness height parameters and selected values of the functional parameters indicating the change in roughness volume according to ISO 25178-2 [67,71] were determined. The changes in the values of the SGT indices were carried out on so-called mapped surfaces, which were not subjected to any additional filtering operations. Each time, the samples were positioned and based in the same way so that changes in the same parts of the surfaces of the samples could be observed in successive series of tests.

3. Results

3.1. SGT Measurements After Milling and After Corrosion Process

The SGT images obtained immediately after the milling operation and after the corrosion process of 20 h are shown in Figure 4, while Table 2 summarises the values for the selected SGT indices.
Salt crystals appeared after the corrosion process, which caused an increase in the number of unmeasured points. The observed qualitative changes in the data are due to the presence of the steep slopes and sharp edges of the salt crystals. It was consciously decided to use an optical system for the measurements, accepting a slight increase in the number of unmeasured points. In contrast, the use of contact methods for the measurements could have led to damage to the delicate structures of the corrosion products on the surface. The nature of the corrosion focus under investigation would then be significantly altered during the measurement, and the values of the SGT indices would be falsified.
Photographs of the surfaces of the corroded samples are shown in Figure 5.

3.2. Changes in SGT Caused by Use of Corrosion Product Removers

The results of the SGT measurements after the two corrosion product removal processes for the selected samples are shown in Figure 6 and Figure 7 and Table 3.
Photographs of the surfaces of the samples after the corrosion product removal processes are shown in Figure 8.
After treatment with corrosion product removers for two hours, the remaining salt crystals were visible. As at the earlier stage of measurement, their presence resulted in an increase in the number of unmeasured points. The observed qualitative changes in the data are due to the existence of the steep slopes and sharp edges of the salt crystals in the corrosion foci.

4. Discussion

4.1. Assessment of SGT After Milling and After Corrosion Process

The corrosion foci, together with the salt crystals, affected changes in the values of the indices describing the surface height, surface functionality and volume. In order to visualise the recorded changes, the data collected in Table 3 are presented as bar charts in Figure 9 and Figure 10.
The corrosion process caused significant individual changes, which increased the height of the surface roughness. This is best seen in the changes in the Sa index values, which increased on average by 66% after the corrosion process. The greatest change was caused by the corrosion process within the reduced peak height. This is illustrated by the Spk index, which changed by 251% on average. The recorded changes within the width of the material core were slightly smaller (the Sk index increased on average by 40%). The smallest changes following the corrosion process were observed in the area of the reduced valley depth (the Svk index increased on average by slightly more than 21%)—see Figure 9. An analogous effect of corrosion on the state of the SGT has also been observed previously [72,73].
The greatest of the changes in the surface volume indices were registered within the peaks of the materials, where new high areas covered by salt crystals appeared after the corrosion process. The Vmp index values increased on average by 65%. The volume of material in the surface core (represented by the Vmc index) increased slightly by 6.3% on average. The emerging salt crystals raising the surface profile also slightly affected the volume of the voids in the surface core. The observed changes in the Vvc index averaged up to 7.7%. The corrosion process slightly changed the shapes and volumes of the valleys. For the Vvv index, the smallest post-corrosion changes were registered, with an average of 1.8% (see Figure 10).

4.2. Assessment of Changes in SGT Caused by Use of Corrosion Product Removers

In order to visualise the recorded changes, the data collected in Table 3 are presented as bar charts in Figure 11 and Figure 12.
On the surfaces of the samples, the corrosion product removers failed to dissolve all corrosion products and salt crystals over a period of 2 h. An increase in the arithmetic mean surface height (Sa) of 9% on average compared to the original roughness height after milling was recorded. This phenomenon is consistent with the results of the work by Parapurath et al. [58] on the surface roughness changes of low-carbon steel samples during etching in HCl solution. The largest contribution was the increase in the reduced peak height (Spk), which additionally reached more than 30% on average. The average recorded change in the reduced valley depth (Svk) was 22%, and the average change in the core roughness depth (Sk) was less than 7.5%. In contrast, one can see the effects of the corrosion product removers on the volumetric surface indices, the values of which were reduced compared to the original values obtained after milling. The largest of the recorded changes relates to an 8% reduction in the average material peak volume values (Vmp). In contrast, the volume changes of the other surface components, i.e., the core material volume (Vmc), core void volume (Vvc) and valley void volume (Vvv), decreased by no more than 5% on average (see Figure 11). This is in line with the results of Sun et al. [62], according to whom it was observed that the surfaces of low-carbon steel samples treated with a neutral corrosion product remover were relatively smoother than the original surfaces.
After 40 h of removing the corrosion products, the effects of the preparations used were much stronger. The corrosion products that accounted for the increase in the arithmetic mean height (Sa), which reached an average of only 1.2% of the initial value obtained after milling, were removed practically in full. However, leaving the metal objects in the corrosion product remover bath for a long period of time also had an effect on the surface layers, resulting in a reduction in the reduced peak heights (Spk) and a deepening of the valleys (represented by the Svk index). These indices changed in relation to the surface indices after milling, decreasing by less than 14% on average. The changes in the volumetric surface indices after a period of 40 h were already relatively small, and their average values did not exceed 3%, apart from the deepening of the volume of the valley voids (represented by the Vvv index, which increased by slightly more than 14% on average)—see Figure 12.
The removal of corrosion products is achieved by targeting residues without affecting the underlying surface structure. This is often facilitated by the special design of the surface texture, which can promote the detachment of the corrosion products while maintaining the structural integrity of the surface [74,75,76,77].
Laser surface texturing is a common method used to enhance corrosion resistance. It creates micro/nanostructures that can trap air or other protective layers, reducing the contact between the corrosive environment and the metal surface [77,78,79]. The textured surfaces can be treated to become hydrophobic or superhydrophobic, which significantly reduces the corrosion rates by preventing water and corrosive agents from adhering to the surface [76,78,79,80,81].
Textured surfaces, especially those treated to be hydrophobic, show an improved corrosion resistance. This is due to the reduced contact area with corrosive agents and the protective barrier formed by trapped air or other substances [76,78,79,81,82]. The ability to remove corrosion products without altering the surface geometry ensures that the functional properties of the surface, such as the wettability and mechanical strength, are maintained [75,76,77,83].
An agent whose action is perfectly aimed at removing corrosion products is the OPN-Rust Remover (R1). It very gently dissolved the corrosion products in a short period of time without modifying the original SGT and stopping further corrosion development. Its action did not change with increasing time and did not expose a new fully metallic surface layer. The surface morphology after 2 h of corrosion product removal (Figure 8a) was very similar to that after 40 h (Figure 8d).
Epoxy Brunox (R2) performed most favourably in removing the corrosion products, clearing all salt crystals from the sample surfaces after two hours and even leading to the digestion of the corrosion products, which were slightly responsible for the reduction in the volumetric values of the SGT indices—see Figure 8b. This agent was the only one that led, as a result of the 40 h test, to the complete removal of all corrosion products, leaving a clean metallic surface (Figure 8e). However, the complete removal of the corrosion products also resulted in significant changes in the heights of the previously created irregularities. The values of the peak and valley volume indices also changed significantly. Although the texture of the surface after the removal of the corrosion products resembled that originally created by milling, the changes (reduction) in the volumes of the irregularities shaping it resembled natural surface wear.
Using the popular cola drink (R3), the active effect of CO2 was observed for about 30 min—see Figure 8c. At the same time, the action of the popular cola drink (R3) slightly resembled that of R1, which blocked the corrosion development mechanism by dissolving the corrosion products and salt crystals in the initial phase of the process. The short-term action of the drink on the surface of the corroded metal led to slight modifications in the volumetric values of the SGT indices. Even so, the new development of previous corrosion foci, resulting in an increase in the surface roughness, was observed (Figure 8f).
In order to better visualise the changes occurring on the surface, the value of the corrosion product removal efficiency index was determined according to this relation:
K S x = S r S m − 1
where Sr—the value of the SGT index after the removal of the corrosion products (within 2 or 40 h), and Sm—the value of the SGT index after milling.
A positive value of the KSx index indicates an increased roughness value compared to the original value recorded on the sample surface after milling, indicating the presence of additional corrosion products on the original metal surface. The most desirable situation is to obtain a value of zero for this index. This means a situation in which the additionally formed corrosion products have been completely removed, and the corrosion product remover has not changed the original SGT to any extent. On the other hand, a negative value of the index indicates an additional violation of the roughness of the metal surface of the sample. In order to better visualise the results obtained, they are presented collectively in a HEATMAP-type table [84] (see Table 4).
The developed table quantitatively confirms the qualitative observations made of the surfaces in the earlier stages. At a removal time of 2 h, the Epoxy Brunox (R2) achieved the average result closest to 0. This is due to the rapid dissolution of the corrosion products across the surface, leading to a change in the shape and height of the roughness. Epoxy Brunox (R2) is a commercial corrosion product converter and primer containing active ingredients such as organic acids (e.g., formic acid) and epoxy-based polymers. The acid component dissolves iron oxides, while the epoxy system forms a protective, passivating layer on the cleaned surface, preventing re-oxidation. This dual action contributes to its superior long-term effectiveness. Surprisingly, the use of the popular cola drink (R3) at 2 h gave better results than the dedicated OPN-Rust Remover (R1). The average value of the efficiency index for removing the corrosion products with the cola drink (R3) was an order of magnitude higher than with the OPN-Rust Remover (R1). The cola drink (R3), in contrast to R1, contains weak organic acids (mainly phosphoric acid) and carbon dioxide, which initially help to dissolve loose corrosion products and mineral salts. The long-term removal of corrosion products produced the opposite result, in which case the OPN-Rust Remover (R1), by removing the corrosion products, did not change the character of the surface. One could say that its effect over time was very gentle. However, it lacks any passivating or protective agent, and its high sugar content and acidity can create residues that attract moisture, which in turn promotes renewed corrosion over time. Furthermore, the CO2 can temporarily acidify the surface micro-environment, accelerating the corrosion once the metal is re-exposed. The popular cola drink (R3), after the initial phase of removing corrosion products in the long term, caused the corrosion processes to develop again, changing the SGT in relation to the originally formed one. Long-term qualitative observations, on the other hand, were not confirmed for the Epoxy Brunox (R2), whose action over time, in addition to removing the corrosion products, resulted in the exposure of the metallic surface, permanently changing the values of all SGT indices.
Table 5 shows the corrosion product removers used in the study, together with their mechanisms of action from the perspective of the corrosion processes.
A comparative analysis of the short- and long-term effects reveals distinct behaviours:
  • In the short term, all three agents (R1, R2 and R3) contributed to the removal of the corrosion products, with the Epoxy Brunox (R2) showing the most pronounced efficiency due to its combined chemical and polymeric actions.
  • In the long term, the Epoxy Brunox (R2) maintained the surface integrity through passivation, whereas the cola drink (R3) led to the formation of new corrosion foci due to the lack of protective properties and residue-promoted moisture retention.
This dual nature of action illustrates the importance of matching the corrosion product remover to the desired functional outcome—immediate cleaning vs. long-term stability.

5. Conclusions

The literature survey carried out and the experimental research completed allow us to formulate the following conclusions:
  • Corrosion and associated problems are an important economic issue. The oft-repeated fact that it is better to use suitable corrosion protection than to carry out corrosion product removal processes afterwards should be emphasised here. Even the smallest foci of corrosion lead to the loss of metallic materials. It is important to detect it at an early stage, when it occupies a small part of the surface and the corrosion is still superficial.
  • If corrosion is already present, it can be removed with common cola drinks. However, their action is limited to short-term exposure and must be followed by immediate rinsing and surface protection, as prolonged contact can lead to further corrosion development. Although the initial effect of cola resembles that of professional agents (such as R1), its long-term effectiveness is significantly lower and may even accelerate the formation of new corrosion foci, increasing the surface roughness. Thus, while cola can be considered as a temporary or emergency solution, it should not be treated as an equivalent alternative to dedicated corrosion product removers.
  • Corrosion product removers left on the surface for too long can lead to permanent changes in the surface. As well as reducing the height of the irregularities, they can lead to changes in the volumetric indices responsible for ensuring the texture and functionality of the object in question.
  • After the application of chemicals to remove corrosion products, it is rare that completely new metallic surface fragments are exposed. The remaining tarnishes and discolourations should be removed additionally in a minimally invasive manner (this was not done during the experimental tests).
  • As a direction for further research, it may possible to check the effects of the hydrophilic properties of a surface on its corrosion resistance by estimating the wetting angles of samples at different measurement stages.

Author Contributions

Conceptualisation, D.G. and A.E.K.; methodology, D.G.; software, D.G.; validation, R.G. and A.E.K.; formal analysis, D.G.; investigation, D.G., A.O. and A.E.K.; resources, D.G. and A.O.; data curation, D.G.; writing—original draft preparation, D.G. and A.E.K.; writing—review and editing, A.O., R.G. and A.E.K.; visualisation, D.G. and R.G.; supervision, A.E.K.; project administration, R.G.; funding acquisition, R.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available upon request from the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this paper:
GDPGross domestic product
SGTSurface geometrical texture

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Coatings 15 00539 g001
Figure 1. Mechanism of corrosion formation and development.
Figure 1. Mechanism of corrosion formation and development.
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Figure 2. Method of shaping SGT during finishing milling operations.
Figure 2. Method of shaping SGT during finishing milling operations.
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Figure 3. Developed experimental plan.
Figure 3. Developed experimental plan.
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Figure 4. SGT images obtained for (a) Sample R1 after milling; (b) Sample R1 after corrosion process; (c) Sample R2 after milling; (d) Sample R2 after corrosion process; (e) Sample R3 after milling; (f) Sample R3 after corrosion process.
Figure 4. SGT images obtained for (a) Sample R1 after milling; (b) Sample R1 after corrosion process; (c) Sample R2 after milling; (d) Sample R2 after corrosion process; (e) Sample R3 after milling; (f) Sample R3 after corrosion process.
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Coatings 15 00539 g005
Figure 5. Photographs of surfaces of samples, with visible salt crystals and corrosion foci: (a) Sample R1; (b) Sample R2; (c) Sample R3.
Figure 5. Photographs of surfaces of samples, with visible salt crystals and corrosion foci: (a) Sample R1; (b) Sample R2; (c) Sample R3.
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Figure 6. The SGT images obtained for (a) Sample R1 after the 2 h corrosion product removal process; (b) Sample R1 after the 40 h corrosion product removal process; (c) Sample R2 after the 2 h corrosion product removal process; (d) Sample R2 after the 40 h corrosion product removal process.
Figure 6. The SGT images obtained for (a) Sample R1 after the 2 h corrosion product removal process; (b) Sample R1 after the 40 h corrosion product removal process; (c) Sample R2 after the 2 h corrosion product removal process; (d) Sample R2 after the 40 h corrosion product removal process.
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Figure 7. The SGT images obtained for (a) Sample R3 after the 2 h corrosion product removal process; (b) Sample R3 after the 40 h corrosion product removal process.
Figure 7. The SGT images obtained for (a) Sample R3 after the 2 h corrosion product removal process; (b) Sample R3 after the 40 h corrosion product removal process.
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Coatings 15 00539 g008
Figure 8. Photographs of the surfaces of the samples: (a) Sample R1 after the 2 h corrosion product removal process; (b) Sample R2 after the 2 h corrosion product removal process; (c) Sample R3 after the 2 h corrosion product removal process; (d) Sample R1 after the 40 h corrosion product removal process; (e) Sample R2 after the 40 h corrosion product removal process; (f) Sample R3 after the 40 h corrosion product removal process.
Figure 8. Photographs of the surfaces of the samples: (a) Sample R1 after the 2 h corrosion product removal process; (b) Sample R2 after the 2 h corrosion product removal process; (c) Sample R3 after the 2 h corrosion product removal process; (d) Sample R1 after the 40 h corrosion product removal process; (e) Sample R2 after the 40 h corrosion product removal process; (f) Sample R3 after the 40 h corrosion product removal process.
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Figure 9. Changes in surface roughness height indices immediately after milling and after corrosion induction.
Figure 9. Changes in surface roughness height indices immediately after milling and after corrosion induction.
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Figure 10. Changes in surface volumetric indices immediately after milling and after corrosion induction.
Figure 10. Changes in surface volumetric indices immediately after milling and after corrosion induction.
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Figure 11. Changes in surface roughness height indices after removal of corrosion products for 2 h and for 40 h.
Figure 11. Changes in surface roughness height indices after removal of corrosion products for 2 h and for 40 h.
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Coatings 15 00539 g012
Figure 12. Changes in surface volumetric indices after removal of corrosion products for 2 h and for 40 h.
Figure 12. Changes in surface volumetric indices after removal of corrosion products for 2 h and for 40 h.
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Table 1. The chemical composition of the OPN-Rust Remover (R1) [63] and Epoxy Brunox (R2) agents [64].
Table 1. The chemical composition of the OPN-Rust Remover (R1) [63] and Epoxy Brunox (R2) agents [64].
OPN-Rust Remover (R1)Epoxy Brunox (R2)
Content, %ComponentContent, %Component
25–50Hydrocarbons, C10–C13, n-alkanes, isoalkanes, cycloalkanes, <2% aromatics10–252-Methoxy-1-methylethyl acetate
10–20Propane10–151-Methoxy-2-propanol
10–20Distillates (petroleum), hydrotreated light naphthenic3–10Isopropyl alcohol
10–20Butane3–10Diethylene glycol monobutyl ether
5–10Isobutane3–10Ethylene glycol
1–2.52-Butoxyethanol1–2Formic acid
Table 2. Values of selected SGT indices of post-milling and post-corrosion samples.
Table 2. Values of selected SGT indices of post-milling and post-corrosion samples.
IndexAfter MillingAfter Corrosion Process
R1R2R3R1R2R3
Sa15.013.59.7020.529.513.4
Spk14.218.312.146.287.422.6
Sk54.342.230.166.570.140.6
Svk7.307.9010.98.3013.010.3
Vmp0.00150.00140.00150.00210.00330.0017
Vmc0.03400.05160.03880.04270.04900.0405
Vvc0.04730.06260.05300.05470.06650.0542
Vvv0.00270.00470.00360.00380.00510.0024
Table 3. Values of selected SGT indices after corrosion product removal processes.
Table 3. Values of selected SGT indices after corrosion product removal processes.
Index2 h Corrosion Product Removal40 h Corrosion Product Removal
R1R2R3R1R2R3
Sa16.014.511.115.29.8013.7
Spk24.917.915.213.98.5016.1
Sk52.847.535.753.431.542.1
Svk7.0013.811.06.509.307.00
Vmp0.00180.00110.00100.00130.00110.0017
Vmc0.04320.03650.04120.04060.03660.0461
Vvc0.05770.04590.05150.05210.04450.0625
Vvv0.00370.00380.00320.00360.00270.0032
Table 4. The values of the corrosion product removal efficiency index.
Table 4. The values of the corrosion product removal efficiency index.
Index2 h Corrosion Product Removal40 h Corrosion Product Removal
R1R2R3R1R2R3
KSa0.070.070.140.01−0.280.41
KSpk0.76−0.020.25−0.02−0.530.33
KSk−0.030.130.19−0.02−0.250.40
KSvk−0.040.760.00−0.100.18−0.36
KVmp0.24−0.16−0.32−0.10−0.150.16
KVmc0.27−0.290.060.20−0.290.19
KVvc0.22−0.27−0.030.10−0.290.18
KVvv0.36−0.18−0.130.30−0.42−0.11
Table 5. Mechanisms of action of corrosion product removers from perspective of corrosion processes.
Table 5. Mechanisms of action of corrosion product removers from perspective of corrosion processes.
AgentType/UseMechanism of ActionShort-Term Effect
on Corrosion		Long-Term Effect
on Corrosion
R1Professional corrosion product removerDissolves corrosion via
solvent penetration, no chemical neutralisation or conversion		Effective removal of loose corrosion products, preserved surface integrityNo passivation—surface susceptible to re-corrosion, protection required after cleaning
R2Industrial corrosion product converter and epoxy primerDissolves iron oxides, forms a protective conversion layer (phosphates, polymers), epoxy acts as barrierEffective corrosion product removal, pre-coating protectionLong-term passivation, possible change in SGT by dissolving raised areas
R3Consumer drink
(used experimentally)		Mild acidic action, dissolves salts and light corrosion products, no protective effect, sugars attract moistureNoticeable short-term cleaning, no full corrosion neutralisationPromotion of re-corrosion, moisture retention due to sugar, increased surface roughness over time
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Ochal, A.; Grochała, D.; Grzejda, R.; Kochmańska, A.E. Investigations on Changes in the Surface Geometrical Texture Caused by the Use of Corrosion Product Removers. Coatings 2025, 15, 539. https://doi.org/10.3390/coatings15050539

AMA Style

Ochal A, Grochała D, Grzejda R, Kochmańska AE. Investigations on Changes in the Surface Geometrical Texture Caused by the Use of Corrosion Product Removers. Coatings. 2025; 15(5):539. https://doi.org/10.3390/coatings15050539

Chicago/Turabian Style

Ochal, Aleksandra, Daniel Grochała, Rafał Grzejda, and Agnieszka Elżbieta Kochmańska. 2025. "Investigations on Changes in the Surface Geometrical Texture Caused by the Use of Corrosion Product Removers" Coatings 15, no. 5: 539. https://doi.org/10.3390/coatings15050539

APA Style

Ochal, A., Grochała, D., Grzejda, R., & Kochmańska, A. E. (2025). Investigations on Changes in the Surface Geometrical Texture Caused by the Use of Corrosion Product Removers. Coatings, 15(5), 539. https://doi.org/10.3390/coatings15050539

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