Assessment of Corrosion Properties of Selected Mineral Waters

: This paper presents an analysis of natural mineral waters recognized by EU member states (on the basis of being announced in the O ﬃ cial Journal of the European Union) and originating in Polish territory. For each of these waters, calculations were made in relation to the Langelier saturation index, Ryznar stability index, and indices S 1 (e ﬀ ect of chloride and sulfate ions on iron and steel), S 2 (e ﬀ ect of nitrate ions on zinc), and S 3 (e ﬀ ect of bicarbonate / sulfate ratio on copper). The impact of mineral waters on copper, hot-dip galvanized iron, stainless steel, acid-resistant steel, cast iron, low-alloy steel, and nonalloy steel was assessed on the basis of the indices calculated. The analysis allowed determination of the possibility of these measures being used in assessing the performance of mineral water installations.


Introduction
Mineral waters, like tap water, are transported to places of collection or use via water pipes. However, they actually demand more from the installation materials used, given the need for the latter to be resistant to aggressive impacts that lead to corrosion. This in turn is reflected by the presence in mineral waters of a higher content of aggressive ions, such as bicarbonate, sulfate, and chloride, as well as free carbon dioxide. Under such conditions, there is a high probability that metal will start to corrode, thereby also causing a deterioration in the quality of the water flowing through it. Where the reaction of water is neutral, metals may be subject to general, pitting, selective, bimetallic, erosive, stress, fatigue, and crevice corrosion [1][2][3].
Although more and more mineral water bottling plants are being built and more and more spas being established where sources of highly mineralized waters are located, there remains a lack of awareness about the need for mineral waters to be treated differently from tap water when it comes to the installation materials used. Therefore, to help designers choose the right installation for a given mineral water, many indices have been developed to help determine, to a greater or lesser extent, water's corrosive properties and its impact on individual metallic installation materials [4][5][6].
Mineral water was first defined at the International Balneological Congress in Bad Nauheim (Germany) in 1911 [7]. For water to qualify as mineral water, its level of mineralization has to be at least 1000 mg/L. The water may also contain natural dissolved gases, such as hydrogen sulfide or carbon dioxide. In most cases, the sources involved lie below impermeable layers (so-called submersible water), with water mineralized through the dissolution of minerals and rocks that it is in contact with.
The classification of mineral waters is as follows [32]: • Low mineral content-a mineral salt content, calculated as fixed residue, not greater than 500 mg/L; • Very low mineral content-a mineral salt content, calculated as fixed residue, not greater than 50 mg/L; • Rich in mineral salts-a mineral salt content, calculated as fixed residue, greater than 1500 mg/L.
Basic regulations on the supply to consumers of water of adequate quantity and quality are transpositions of the Drinking Water Directive (Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption), with its latest amendment including Commission Directive (EU) 2015/1787 of 6 October 2015 [33]. This specifies permissible concentrations of individual elements that classify water from taps in terms of drinking water.
Bottled mineral waters analyzed in this work are contained in the aforementioned Official Journal of the European Union [17]. This posting is in compliance with Article 1 of Directive 2009/54/EC of the Parliament and of the Council dated 18 June 2009 [32].

Materials and Methods
This section provides details of the proposed methods to assess the corrosive properties of selected mineral waters. The indices used to assess the stability of water were as follows: the value of pH at saturation, the Langelier index I L , the Ryznar index I R , the Larson and Skold index S 1 , the S 2 index of selective corrosion in iron pipes that are hot-dip galvanized, and the S 3 index for the assessment of pitting corrosion in pipes made of copper [22][23][24][25][26]. The analysis was carried out with Statsoft software [34].

Description of the Study Region
The mineral waters considered in this study are mined in Poland, a country located in Central Europe and on the Baltic Sea. It has a humid continental climate, though one that is also defined as transitional between warm and rainy temperate and snow-forest boreal. Poland is inhabited by 38.5 million people over an area of 312,700 km 2 , with an average population density of 123 people/km 2 . Poland occupies the area between 54 • 50 and 49 • 00 N and 14 • 07 and 24 • 09 E ( Figure 1). Because lowland areas (below 200 m a.s.l.) predominate in Poland (accounting for as much as 75% of the country's area), the average height is only 173 m a.s.l., and the median height is 149 m a.s.l. The lowlands occur in the north and in the center, with mountain and upland areas in the south.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 15 The classification of mineral waters is as follows [32]: • Low mineral content-a mineral salt content, calculated as fixed residue, not greater than 500 mg/L; • Very low mineral content-a mineral salt content, calculated as fixed residue, not greater than 50 mg/L; • Rich in mineral salts-a mineral salt content, calculated as fixed residue, greater than 1500 mg/L.
Basic regulations on the supply to consumers of water of adequate quantity and quality are transpositions of the Drinking Water Directive (Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption), with its latest amendment including Commission Directive (EU) 2015/1787 of 6 October 2015 [33]. This specifies permissible concentrations of individual elements that classify water from taps in terms of drinking water.
Bottled mineral waters analyzed in this work are contained in the aforementioned Official Journal of the European Union [17]. This posting is in compliance with Article 1 of Directive 2009/54/EC of the Parliament and of the Council dated 18 June 2009 [32].

Materials and Methods
This section provides details of the proposed methods to assess the corrosive properties of selected mineral waters. The indices used to assess the stability of water were as follows: the value of pH at saturation, the Langelier index IL, the Ryznar index IR, the Larson and Skold index S1, the S2 index of selective corrosion in iron pipes that are hot-dip galvanized, and the S3 index for the assessment of pitting corrosion in pipes made of copper [22][23][24][25][26]. The analysis was carried out with Statsoft software [34].

Description of the Study Region
The mineral waters considered in this study are mined in Poland, a country located in Central Europe and on the Baltic Sea. It has a humid continental climate, though one that is also defined as transitional between warm and rainy temperate and snow-forest boreal. Poland is inhabited by 38.5 million people over an area of 312,700 km 2 , with an average population density of 123 people/km 2 . Poland occupies the area between 54°50′ and 49°00′ N and 14°07′ and 24°09′ E ( Figure 1). Because lowland areas (below 200 m a.s.l.) predominate in Poland (accounting for as much as 75% of the country's area), the average height is only 173 m a.s.l., and the median height is 149 m a.s.l. The lowlands occur in the north and in the center, with mountain and upland areas in the south.

Characteristics of the Research Object
Data on the content of individual ions were collected from individual bottling plants extracting a given water, with about 78 samples taken into account. Table 1 presents some of the analyzed bottled mineral waters available in Poland for retail and wholesale. These are extracted from both spa areas and other places in Poland. All properties of selected bottled mineral waters are contained in the table in Appendix A (Table A1)

Indices Assessing the Corrosive Effect of Water on Water Installations
The corrosive effect of water on water installations was assessed using indices as follows: • Water stability, i.e., the pH value at saturation, defined as follows: where A is a value determined on the basis of dry residue, B is a value determined on the basis of temperature, C is a value determined in reference to calcium content, and D is a value determined in reference to total alkalinity.
The values necessary to determine the pH of water at saturation are contained in the table in Appendix A (Table A2).

•
The Langelier index, as used to assess the corrosive effect of water on steel and galvanized installations, calculated in line with a formula in the standard [34] as follows: where I L is the saturation index, pH 0 is the pH of the examined water sample, and pH s is the pH in the saturated state.
The saturation index often refers to the likelihood of determining limestone carbonate formation. The index assumes that, in the case of sediment formation, the rate of the corrosion process is low, meaning that water is only slightly corrosive. Sedimentation will occur if the indicator is positive. However, if the index is negative, it means that the calcium compounds in the water are dissolved and the corrosiveness of the water is high. If the index value is zero, the water has poor corrosion properties and there is a possibility of lime scale.

•
The Ryznar index, based on the same assumptions as I L and otherwise known as the Ryznar stability index. The method of calculation combines analytical data with theories about the saturation of water in calcium carbonate, with the aim of predicting the tendency for sediment to form as well as the corrosiveness of water relative to carbon steel. The Ryznar index is calculated in line with the following formula: where I R is the stability index.
Different values for the index correspond to different levels of stability of water [35]. When the value is below 6, the stability of water is deemed to be lower given the associated deposition of CaCO 3 sediment and hence has more limited corrosiveness [36]. In turn, when the indicator value is in the 6-8 range, water is probably unstable and there is a possibility of underlying corrosion. However, if the I R value exceeds 8, the water is stable and its corrosiveness increases. To summarize, under the Ryznar index, stable water is that characterized by a value maintained in the range 6.25-6.75.

•
The Larson and Skold index, used to assess the effect of chloride and sulfate ions on an installation, is expressed by the following formula: If the value of S 1 is greater than 1, there is a likelihood of accelerated local corrosion of low-alloy iron materials. This is due to oxygen content greater than 0.1 g O 2 /m 3 . If the coefficient attains a value greater than 3, this means that the water has corrosive properties vis-à-vis galvanized steel, with possible local corrosion.

•
The S 2 indicator assessing the occurrence of selective corrosion in hot-dip galvanized iron pipes, as used to assess the possibility of intercrystalline corrosion of zinc, where water has an elevated content of NO 3 − ions, and based on the following formula: Selective leaching of zinc is present when the value of this ratio is below 2. The phenomenon occurs along the grain boundaries of zinc.

•
The S 3 evaluation index for the occurrence of pitting corrosion in pipes made of copper, used to assess corrosiveness in the case of an installation made of copper and described by the following formula: The risk of copper-pitting corrosion is high when the value of the indicator is under 2. Table 2 summarizes the criteria used in assessing the corrosion resistance of individual materials. Acid-resistant steel has the most limited requirements, with indices being basically of no significance as only the content of chloride ions is important. At the other extreme, hot-dip galvanized iron is problematic, given the need for matches in the case of 4 of the 6 indices.

Criteria for Assessing the Corrosion Resistance of Individual Materials
An additional criterion for assessing the corrosion resistance of materials is their alkalinity. The water of alkalinity below 1.36 mol/m 3 is aggressive, regardless of the values assumed by other indices. In the case of the H 2 S parameter, there is a lack of guidelines as to the value for the indices at which corrosion of individual installation materials will not occur. Table 2. Values of indicators at which corrosion of individual installation materials will not occur (based on [22][23][24][25][26]35,36]).

Type of Installation Material
Parameter Copper and its alloys -

Corrosiveness Indices for Bottled Mineral Waters
Using the formulae presented in Section 3.3.1. The following corrosion indices were calculated and are presented in Figure 2.

Type of Installation Material
Parameter IL IR S1 S2 S3 Copper and its alloys -

Corrosiveness Indices for Bottled Mineral Waters
Using the formulae presented in Section 3.3.1. The following corrosion indices were calculated and are presented in Figure 2. In the case of the (S2) index assessing the occurrence of selective corrosion in hot-dip galvanized iron pipes, 12.8% of all tested waters obtained results above 1.84, with a maximum of 1300.35 reached.

Results for Assessment of the Resistance of Installation Materials to the Corrosive Effects of Mineral Waters
Based on the collected data and the results of corrosion indicators, an assessment was made of the resistance of installation materials to the corrosive effects of the mineral waters discussed in this In the case of the (S 2 ) index assessing the occurrence of selective corrosion in hot-dip galvanized iron pipes, 12.8% of all tested waters obtained results above 1.84, with a maximum of 1300.35 reached.

Results for Assessment of the Resistance of Installation Materials to the Corrosive Effects of Mineral Waters
Based on the collected data and the results of corrosion indicators, an assessment was made of the resistance of installation materials to the corrosive effects of the mineral waters discussed in this work. The materials evaluated were copper, galvanized iron, stainless steel, acid-resistant steel, cast iron, low-alloy steel, and nonalloy steel. Table 3 presents the assessment of the resistance of installation materials to bottled mineral waters. All values for the indices are contained in the table in Appendix A (Table A3). Table 3. Values for indices at which corrosion of individual installation materials will not occur-a snapshot of all the waters analyzed.

Copper Galvanized Iron Stainless Steel Acid-Resistant Steel Cast Iron Low-Alloy Steel Nonalloy Steel
means the possibility of using installation materials, and X means the lack of such possibility.
Acid-proof steel emerged as the most comprehensive material as it could be used in 33% of all cases. Following it in the list were iron (17%) and copper (15%). The materials with the lowest use were nonalloy steel (3%) and hot-dip galvanized iron (8%).
The most typical installation materials for water supply systems are polyvinyl chloride (PVC), polyethylene (PE), and polybutylene (PB) plastics. Fiberglass pipes are also used in the case of thermal waters. Plastics have gained widespread use due to the absence of corrosiveness as a criterion underpinning their selection. However, components of the above polymer installations are seen as a source of nutrients for bacteria found in water transmission systems. The result of their metabolic activity is the formation of a biofilm in the internal parts of ducts, which then assume the appearance of having been affected by microbial corrosion [37,38]. Previous studies [15,39] have indicated the importance of the selection of pipe material in water systems with high corrosiveness, among others, on the basis of water quality parameters, such as pH, alkalinity, and calcium hardness. The presented approach has been proven to be suitable for assessing the corrosive properties of mineral waters, as confirmed by analysis shown in [40,41]. The mentioned research shows the need for the broad characterization of mineral water compositions, including their aggressiveness concerning different materials, using the analyzed indices, which in turn will help to establish criteria for categorizing stability of the water [42,43]. The development of material science is dynamic, and its effect is the production of increasingly new materials, which are systematically introduced in the construction of water supply networks [44,45]. When introducing new materials for the construction of water supply networks, it should be taken into account that these materials require a completely new approach at the planning stage of construction, the construction itself, or during testing of the condition of the water supply network in comparison with the materials used so far [12]. Such tests take time and only after a long period of use will it be possible to determine exactly to what extent they affect the quality of the transported water.
Plastics are prone to fouling by microorganisms, so those choosing this type of installation for aggressive mineral waters will need to recall the possibility of secondary microbial contamination and the appearance of corrosion.
Coatings 2020, 10, x FOR PEER REVIEW 8 of 15 Acid-proof steel emerged as the most comprehensive material as it could be used in 33% of all cases. Following it in the list were iron (17%) and copper (15%). The materials with the lowest use were nonalloy steel (3%) and hot-dip galvanized iron (8%).
The most typical installation materials for water supply systems are polyvinyl chloride (PVC), polyethylene (PE), and polybutylene (PB) plastics. Fiberglass pipes are also used in the case of thermal waters. Plastics have gained widespread use due to the absence of corrosiveness as a criterion underpinning their selection. However, components of the above polymer installations are seen as a source of nutrients for bacteria found in water transmission systems. The result of their metabolic activity is the formation of a biofilm in the internal parts of ducts, which then assume the appearance of having been affected by microbial corrosion [37,38]. Previous studies [15,39] have indicated the importance of the selection of pipe material in water systems with high corrosiveness, among others, on the basis of water quality parameters, such as pH, alkalinity, and calcium hardness. The presented approach has been proven to be suitable for assessing the corrosive properties of mineral waters, as confirmed by analysis shown in [40,41]. The mentioned research shows the need for the broad characterization of mineral water compositions, including their aggressiveness concerning different materials, using the analyzed indices, which in turn will help to establish criteria for categorizing stability of the water [42,43]. The development of material science is dynamic, and its effect is the production of increasingly new materials, which are systematically introduced in the construction of water supply networks [44,45]. When introducing new materials for the construction of water supply networks, it should be taken into account that these materials require a completely new approach at the planning stage of construction, the construction itself, or during testing of the condition of the water supply network in comparison with the materials used so far [12]. Such tests take time and only after a long period of use will it be possible to determine exactly to what extent they affect the quality of the transported water.
Plastics are prone to fouling by microorganisms, so those choosing this type of installation for aggressive mineral waters will need to recall the possibility of secondary microbial contamination and the appearance of corrosion.

Conclusions and Perspectives
To this date, there have been no legal recommendations regarding assessment of the corrosiveness of water by supply companies, spas, and bottling plants. Such a procedure could do much to help designers choose the right installation without exposing the client to additional operating costs and even the need to replace corrosion-damaged pipes. There would certainly be a positive effect on the lifespan of pipes as well as the quality of the water itself.
This work makes it clear that a duty to assess corrosiveness should be imposed by law on institutions distributing mineral waters. Polish mineral waters are aggressive waters, as evidenced by the fact that 8% of the types considered were too aggressive to gain use in association with the materials analyzed. Equally, it needs to be recalled that the use of cheaper and theoretically noncorrosive materials may emerge to be uneconomical and may indeed give rise to entirely unexpected phenomena.
Operators should be involved in the design of water distribution systems and installations using appropriate materials to ensure an adequate level of safety from the water source to the recipient. It should be noted that it is necessary to adapt the internal material of the water supply system to the water parameters. There is currently no correlation between the design phase and the water parameters. It was found that in order to protect the water infrastructure, which is a critical infrastructure, water supply company should place more emphasis on the distribution of stable water that has no potentially corrosive properties. In perspective, some suggestions will be made regarding protection of the water distribution system and its safe operation as well as the long-term durability of water supply pipes.
Author Contributions: All authors equally contributed to the development of this manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Acknowledgments:
We thank the reviewers for their feedback, which helped to improve the manuscript quality.

Conflicts of Interest:
The authors declare no conflict of interest.