376 Journal homepage: www.fia.usv.ro/fiajournal Journal of Faculty of Food Engineering, Ştefan cel Mare University of Suceava, Romania Volume XIII, Issue 4- 2014, pag. 376 - 384 NEW FRI END LY ENV IRON ME NTA L ETCHI NG T ECHNO LOG IES FOR SOM E ST E ELS AN D CO PPER A LLOYS Vira VODYANKA1, Sergiy BORUK2, *Igor WINKLER2 1Kyiv National University of Commerce and Economy, Chernivtsi College of Commerce, vera-79@ukr.net 2Yu. Fedkovych National University of Chernivtsi, Ukraine, i.winkler@chnu.edu.ua *Corresponding author Received November 18th 2014, accepted December 29th 2014 Abstract: New compositions containing some organic inhibitors were tested in the carbon steels and copper alloys etching processes. The analysis of the etching efficiency and harmful products emission proves that significant reduction in the specific discharge of the waste waters and emission of nitrogen oxides can be achieved at the appropriate etching quality. This reduction is caused mainly by adsorption of the organic inhibitors on the metal surface by de- creasing its excessive dissolution. On the other hand, the effective aggregation and dispersion of the corrosion products promotes faster transportation of the rust and dross particles away from the treat- ed metal surface reducing the total processing time. Keywords: environmental safety; corrosion; etching; carbon steels, copper alloys 1. Introduction The galvanic industry is the second highest pollution source after the surface transport branch and it causes continuous and long- lasting contamination of the entire envi- ronment (air, water and soil). Therefore, the extensive efforts should be directed to prevent this dangerous contamination bringing many aggressive components of the galvanic (including metals etching) composition into the enevironment [1-7]. Among the others, the metal etching tech- nologies produce significant amounts of the air and water pollution agents and ex- tended activity is directed onto investiga- tion of possible mitigation steps, which should provide same etching quality and efficiency at lower emission of pollutants. Sulfuric, hydrochloric and nitric acids are used as the main components for the low- alloy carbon steel etching compositions [2- 3]. Significant amounts of iron can dis- solve in sulfuric acid during this process while dissolution of iron in the hydrochlo- ric acid compositions is less influential. On the other hand, a rate of the metal rust and scale dissolution in the latter compositions is higher than in the former, therefore, the hydrochloric compositions are more suita- ble for the low-allow carbon steel etching [2, 3, 8]. An optimal content of sulfuric acid in the regular etching compositions is 20-22 %; hydrochloric acid – 18-20 %. Additional organic substances are usually added to the both types of the composi- tions to counteract excessive metal dissolu- tion, decrease emission of the gaseous pol- lutions agents and minimize amount of the wastewater formed. Thiocarbamide- hydrosulfite-, sulfite-, thi- osulfate- and thiocyanate compounds are used regularly in the nitric acid based etch- ing compositions in order to protect the metal surface from its dissolution. Howev- er, these compounds are quite toxic and Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 377 can provide effective surface protection within quite a narrow pH range only [2, 3, 8-11]. It is obvious, that such inhibitors should be substituted with different, less toxic sub- stances with the wider working pH range. The phosphoric acid etching compositions are also used in industry. Trace amounts of the unwashed acid can provide additional corrosion protection since iron phosphates are insoluble and form quite dense surface protection film. On the other hand, this ac- id is weaker and effective etching requires the solution to be heated up to 85 0C, caus- ing rise in the processing cost. Due to this shortcoming, the phosphoric compositions have quite limited usage and are recom- mended mostly for the fine etching of the surfaces to be dyed. A number of the mixed acids compositions are also used in the modern industry for etching [2, 3]. Two types of compositions are used for the chemical polishing of copper and its al- loys. The first type consists of various mix- tures of nitric (5-20 vol. %), phosphoric (30-80) and acetic (10-50) acids and the second type is based on hydrogen perox- ide. The higher is concentration of nitric acid, the worse is the surface glitter while efficiency of the polishing decreases with rise of the phosphoric acid content. Exces- sive concentration of the acids may result in the unwanted etching process running along with the polishing. Most polishing compositions are effective only for the temperatures under 323-373 K and for the copper contents below 30-36 g/l [2, 3 12- 14]. This process is unsafe environmental- ly and causes emission of harmful nitrogen oxides [2, 3]. Various nitrogen- and sulfur-containing substances can be used in the polishing technology as inhibitors. For instance, urotropine is widely used in various com- posite inhibitors together with catapine, tribenzenamine or thiocarbamide. The former substance exhibits own inhibi- tion activity and promotes higher activity of the other components. However, such compositions are used at the temperatures above 298 K, which increases the produc- tion cost. As reported in [15-19], some ni- trogen-containing heterocycles (derivatives of quinoline, pyridine, acrydine, indole and others) can provide an effective inhibition for the acidic polishing mixtures. Howev- er, development of specific polishing com- positions is still required in various indus- trial branches. This paper deals with investigation of some new effective, nontoxic, environ- ment-friendly, inexpensive compositions for the acidic etching of the carbon steels, copper and its alloys, which produce re- duced amounts of the harmful products. As stated above, the carbon steel etching compositions are usually based on the mix- ture of hydrochloric and nitric acids. As the latter agent is the strong oxidizer, the proposed inhibition compositions should remain active and stable in such aggressive solution. Two or more different inhibition agents are usually mixed in various compositions [2, 3. 14-19] in order to cover all technologi- cal range of temperatures, pH and other parameters of the solutions. The protective activity of the compositions and the range of their usability can be changed by selection of various concentra- tions of the components. On the other hand, changes in these con- centrations would also influence the tech- nological characteristics of the etching process and emission of the harmful com- pounds. Therefore, such an investigation should be aimed onto finding of the environmental friendly compositions ensuring high tech- nological efficiency Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 378 2. Experimental Two groups of parameters have been measured in order to evaluate influence of the admixtures on the process of the metal surface etching. A: Environmental parameters: specific emission of nitrogen oxides; concentration of metal ions in the wastewater; specific amount of the wastewaters formed; B: Technological parameters: etching quality; mass loss and rate of the metal dis- solution. All experiments were carried out at 293±2 K during 5 minutes (for the steel samples) or 30 seconds (copper and copper alloys samples) using the etching compositions with two different inhibitors. The first in- hibitor was taken from the group I: ben- zenesulfonic acid; sulfosalicylic acid; sul- fanilic acid while the second agent was taken from the group II: thiosemicarba- zide; acetylcarbamide; pyrimidone; car- bamide; N-n-etoxyphenylcarbamide. The most optimal concentrations of the inhibitor I and inhibitor II for the carbon steels St10 and St3 were sought for the fol- lowing acidic etching composition (1) (g/l): Hydrochloric acid (ρ = 1.19 g/ml) 200; Nitric acid (ρ = 1.34 g/ml) 120; Inhibitor I up to 10 Inhibitor II up to 2.0. As seen from the electrochemical data (see Table 1, 2), the majority of the group II inhibitors show good efficiency for etching the St10 steel samples in the composition (1) with sulfosalicylic acid (10 g/l). The group I inhibitors exhibit the mixed type of activity and can decelerate both cathodic (hydrogen release) and anodic (metal dis- solution) processes (see Table 1). A tech- nologically sufficient metal protection can be ensured at 293 K by adding of 2 g/l of acetylcarbamide or 0.2 g/l of thiosemi- carbazide together with 10 g/l of sulfosali- cylic acid to the composition (1) (Table 2). For example, the corrosion retardation co- efficient γ = 4,8 and 15.0 and the metal surface protection degree was 79.2 % and 93.3 % for acetylcarbamide and thiosemi- carbazide simultaneously. Since effective corrosion protection re- quires γ ≥ 8 or the metal protection degree over 87 %, thisosemicarbazide can be clas- sified as very effective inhibitor while effi- ciency of acetylcarbamide is lower but still sufficient. Both substances contain atoms of nitrogen and oxygen (or sulfur) with the opposite effective charges, which facilitates possi- ble adsorption on both anodic and cathodic areas on the metal surface and formation of the dense adsorption layer. Sulfosalicylic acid provides additional retardation of the metal dissolution because it contains the functional groups SO3H- і OH-, which ex- hibit some inhibition effect on the anodic areas. All these properties ensure good inhibition effect and prove the assumption of higher efficiency of the two-component inhibitors protecting the metal surface during its etching [2, 3]. Table 1 Some electrochemical parameters of the St10 steel etching processes with or without inhibitors of the group I Inhibitor і,A/dm2 φs, V Z, % γ Сa, % Wc, g/mm2∙year Source composition (no inhibitor used) 0.024 0.391 - - 40 1.68 Benzenesulfonic acid 0.017 0.370 29.17 1.4 33 1.16 Sulfosalicylic acid 0.016 0.382 33.3 1.5 37.5 1.12 Sulfanilic acid 0.014 0.382 41.7 1.7 44 0.96 Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 379 3. Results and discussion The inhibition efficiency has been estimat- ed (see Tables 2, 3) using the following parameters: i – current density (A/dm2); φs – potentials of the metal sample surface (V); Wc – index of the metal corrosion dis- solution (g/mm2 hour); Z – inhibition ac- tivity coefficient (%); γ – corrosion retar- dation coefficient; Ca – anodic control de- gree (%). Current density (i) and potentials of the metal sample surface (φs) were determined through direct instrumental measurements. Index of the metal corrosion dissolution (Wс) was calculated by the formula: Wс = ∆m /(S×t), where ∆m – change in the steel or copper sample mass, g; t – duration of the process, h; S – surface area of the metal sample, m2 (we used samples with S = 0.1 m2); Wс – rate of the metal surface dissolution, g/m2·h. The value (Z) – metal surface protection degree characterizes depth of the corrosion retardation and can be calculated by the formula: %100 0 0      Z , where  and 0 – corrosion rates with and without inhibitor simultaneously. The dimensionless corrosion retardation coefficient γ shows the ratio of the corro- sion rate retardation by the given inhibitor and can be calculated as: ,0    where  and 0 are similar to the formula above. Anodic control degree (Ca) was cal- culated by the formula: aC = %100 0E Ea   , where aE – is a shift of the working an- odic potential from its initial value under the given electric current; 0E – the initial potentials difference between the cathodic and anodic areas. Table 2 Some electrochemical parameters of the St10 steel etching process in the solution consisting sulfosalicylic acid and some inhibitors of the group II Inhibitor і,A/dm2 φs, V Z, % γ Сa, % Wc, g/mm2∙year 1 g/l acetylcarbamide 0.014 0.362 41.7 1.7 50 0.96 2 g/l acetylcarbamide 0.005 0.380 79.2 4.8 50 0.36 1 g/l carbamide 0.01 0.396 58.3 2.4 50 0.68 2 g/l carbamide 0.006 0.397 75.0 4.0 50 0.4 1 g/l N-n- etoxyphenylcarbamide 0.018 0.376 25.0 1.3 50 1.24 2 g/l N-n- etoxyphenylcarbamide 0.02 0.375 0.17 1.2 50 1.4 1 g/l pyrimidone 0.01 0.382 58.3 2.4 67 0.68 2 g/l pyrimidone 0.009 0.391 62.5 2.7 60 0.64 0.1 g/l thiosemicarbazide 0.002 0.397 91.7 12.0 50 0.12 0.1 g/l thiosemicarbazide 0.0016 0.398 93.3 15.0 60 0.12 Table 3 represents some parameters of St10 and St 3 etching in the acidic compo- sition (1) with some inhibitors of the group I. As seen from Table 3, efficiencies of benzenesulfonic acid and sulfosalicylic acid are similar and higher than efficiency of sulfanilic acid. Therefore, the two for- mer agents can ensure more efficient etch- ing with better environmental parameters. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 380 Since sulfosalicylic acid is cheaper and used widely in the industry, it was selected for our further investigations as the most effective inhibitor of the group I. Table 3 Some parameters of St10 and St 3 etching in the composition (1) with some inhibitors of the group I (*) Inhibitor C. g/l (Δm.mg)/(d. μm) Wc. g/m 2·h VNOx. mg/l Vs. l/m 2 Composition (1). no inhibitors - 98/1.4 117.6 112.3 38 Benzenesulfonic acid 10.0 28/0.40 33.6 52.9 14 Sulfosalicylic acid 10.0 29/0.41 34.8 41.7 15 Sulfanilic acid 10.0 51/0.73 61.2 71.3 24 (*) C – inhibitor concentration in the etching composition; Δm – changes in the sample mass after etching; d – thickness of the dissolved metal layer; Wc – rate of the metal dissolution; VNOx – specific emission of NOx; Vs – specific volume of the wastewater formed. An influence of the group II agents togeth- er with sulfosalicylic acid has been inves- tigated at the next stage (see results in Ta- ble 4). Table 4 Some parameters of St10 and St3 steels etching in the composition (1) with sulfosalicylic acid and the group II agents (*) Inhibitor C. g/l (Δm.mg)/(d. μm) Wc. g/m 2·h VNOx. mg/l Vs. l/m2 Carbamide 2.0 20/0.29 24.0 24.3 10.0 Acetylcarbamide 2.0 7/0.10 8.4 4.5 3.5 Pyrimidone 2.0 10/0.14 12.0 5.6 5.0 N-n-etoxyphenylcarbamide 1.5 42/0.60 50.4 34.4 21.0 Thiosemicarbazide 0.2 3/0.04 3.6 3.75 3.0 (*) All notations are similar to those in Table 3. The compositions with acetylcarbamide (2.0 g/l) and thiosemicarbazide (0.2 g/l) together with sulfosalicylic acid prove the best inhibition efficiency since their inhibi- tion and dispersing properties ensure better environmental and technological parame- ters of surface treatment of the carbon steels, copper and its alloys (see Table 4 and 5). Specific emission of the nitrogen oxides reduces tenfold (from 0.0082 to 0.00048 mg/m3 for the carbon steel etching and down to 0.088 mg/m3 for the copper alloys). Amount of the wastewaters formed reduc- es 3-4 times, the rate of the metal dissolu- tion decreases by one order of magnitude and quality of the surface after the treat- ment becomes significantly higher compar- ing to the results achieved by the tradition- al technologies. Therefore, the following two compositions can be recommended for the carbon steels etching: Table 5 The compositions recommended for the carbon steels etching Composition №1 (g/l): Composition №2 (g/l): Composition №3 (g/l): Hydrochloric acid (ρ=1.19 g/ml) – 200; Nitric acid (ρ=1.34 g/ml) – 120; Sulfosalicylic acid – 10; Acetylcarbamide – 2.0; Distilled water – up to 1 l Hydrochloric acid (ρ =1.19 g/l) – 200; Nitric acid (ρ =1.34 г/см3) – 120; Sulfosalicylic acid – 10; Thiosemicarbazide – 0.2; Distilled water – up to 1 l Phosphoric acid (ρ = 1.834 g/ml) – 275; Acetic acid (ρ = 1.049 g/ml) – 125; Nitric acid (ρ = 1.340 g/ml – 100; Thiosemicarbazide – 0.15; Distilled water - up to 1 l. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 381 A series of the thioamides derivatives was checked for their inhibition efficiency in the copper and copper alloys etching and polishing (see Table 6) and phenylthio- semicarbazide and thiosemicarbazide (0.15 g/ml) have shown the best inhibition and brightening performance. These substances also facilitate the significant reduction in nitrogen oxides emission and in amount of the wastewaters formed. Table 6 Some parameters of the copper alloys polishing with and without inhibition agents (*) Inhibition agent С. g/l Δm. mg / d. μm Wc. g/sm2·hour VNOx. mg/l Vs l/m 2 ∆. % Polishing mixture. no inhibitor - 430/6.14 860.0 181.0 18.2 +2 Thiocarbamide 0.15 250/3.57 500.0 76.1 4.0 +15 Thioacetamide 0.15 190/2.71 380.0 65.0 3.0 +4 Rubeanic acid 0.15 170/2.43 204.0 59.0 12.5 -7 Thiosemicarbazide 0.15 130/1.86 260.0 7.6 0.65 +17 Phenylthiosemicarbazide 0.15 115/1.64 230.0 7.5 0.6 +12 (*) All notations are similar to those in Table 3. ∆ means changes in the surface glitter. %. Therefore, the following composition can be recommended for polishing of copper and its alloys (column 3 in Table 5) . A character and nature of the inhibition activity of various agents can be under- stood from analysis of changes in the asso- ciative interaction between the group I and group II compounds. This interaction caus- es some changes in the spectral parameters of the solution described in details below. At the next stage, an influence of the cor- rosion inhibitors (sulfosalicylic acid. thio- semicarbazide, carbamide, acetylcar- bamide and pyrimidone) on environmental safety and performance of the metal etch- ing compositions has been investigated. Special attention has also been given to the predicted associative interaction between the group I agent (sulfosalicylic acid) and the group II agents (thiosemicarbazide, carbamide, acetylcarbamide and pyrim- idone). Results of this investigation are represent- ed in the Tables 7 and 8. It can be seen that pyrimidone and thiosemicarbazide do not form any associates with sulfosalicylic acid since the characteristic light absorbance maximum of sulfosalicylic acid does not shift after mixing with both agents (see Tables 6 and 7). Therefore, these inhibitors adsorb on the metal surface independently. In contrary, formation of the associates be- tween sulfosalicylic acid and carbamide has been determined and position of the light absorbance peak shifts for about 80 nm – from 250 to 330 nm (See Fig. 1). Sulfosalicylic acid and acetylcarbamide also form associates causing the acetylcar- bamide maximum absorbance peak in the mixture to shift for 55 nm in comparison to the pure acetylcarbamide solution (see Fig. 2). Table 7 Position of the light absorbance peak and its intensity for sulfosalicylic acid, pyrimidone and their mixture Compound λmax Lgε Sulfosalicylic acid 337.5 4.83 Pyrimidone and sulfosalicylic acid 337.5 4.90 Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 382 Table 8 Position of the light absorbance peak and its intensity for sulfosalicylic acid. thiosemicarbazide and their mixture Compound λmax Lgε Sulfosalicylic acid 337.5 4.83 Sulfosalicylic acid and thiosemicarbazide 338.0 4.79 The above mentioned associates are com- paratively instable since the shifted ab- sorbance peaks were found returned to the previous positions in 2-3 hours. For in- stance. the pure acetylcarbamide spectral data showed the peak in the position 1. admixture of sulfosalicylic acid caused its shifting to the position 2 and then the peak was recorded in the position 1 again in 2-3 hours (see Fig. 1). Since C/A dependence (C – adsorbate concentration; A – adsorption value) on adsorbate concentration is linear (see Fig. 3). the process of adsorption can be represented by the monomolecular Lang- muir mechanism. Fig.1. Dependence of the optical density (D) on wave- length (λ) for aqueous solutions of carbamide. and its mixture with salicylic acid; 1 – carbamide; 2 – mixture of sulfosalicylic acid and caramide (concentration of both compounds is 1•10-5 m/l). Fig. 2. Dependence of the optical density (D) on wavelength (λ) for aqueous solutions of acetylcar- bamide. and its mixture with sulfosalicylic acid; 1 – acetylcarbamide; 2 – mixture of sulfosalicylic acid and acetylcaramide (concentration of both compounds is 1•10-5 m/l). As seen from Fig. 4. sulfosalicylic acid adsorption on the iron oxides powder is higher than on iron and steel. Similar com- parison for acetylcarbamide proves its bet- ter adsorption on iron, then on steel and iron oxides (see Fig. 5). As seen from the Fig. 4 and 5, according to the descending adsorption activity in relation to sulfosalicilic acid (a) and acetylcarbamide (b), the adsorbents can be arranged in the following sequences: (a): FeO > Fe2O3 > Fe > St10 (b): Fe > St10 > FeO > Fe2O3. It was found that the descending adsorp- tion activity sequence related to carbamide. pyrimidone and thiosemicarbamide is simi- lar to the row (a). Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 383 Fig. 3. Near-to-linear dependencies of C/A on the adsorbate (sulfosalicilic acid) concentration proves the Langmiur mechanism of its adsorption on the following samples: steel St10 (1); iron powder (2); Fe2O3 powder (3) and FeO powder (4). Fig. 4. Adosrption isotherms of sulfosalcylic acid on: 1 – FeO. 2 – Fe2O3. 3 – Fe. 4 – steel St10. Effective aggregation and dispersion of the corrosion products are essential to ensure higher environmental safety of the etching composition. Fast aggregation and dispersion of the rust and dross particles promote faster transportation of these products away from the metal surface, increase in the etching surface area and in general, higher rate of the process. Therefore, total etching time decreases causing reduced emission of the hazardous etching gases. In this context, an influence of sulfosalicylic acid, thiosemicarbazide and their mixtures on the aggregation stability of the aqueous suspensions containing iron oxides and some corrosion products has been investigated. Fig. 5. Adosrption isotherms of acetylcarbamide on: 1 – Fe. 2 – steel St10. 3 – FeO. 4 – Fe2O3. Fig. 6. Dependence of the sedimentation stability coefficient (K) on the concentration of sulfosali- cylic acid for: 1 – St10 steel corrosion products; 2 – particles of Fe2O3; 3 – particles of FeO. The data of Fig. 6 prove that increase in the inhibitor concentration results in higher sedimentation stability. An optimal etching mode can be achieved at some ratio between two inhibitors (sulfosalicylic acid and acetylcarbamide or sulfosalicylic acid and thiosemicarbazide) when the inhibitors provide sufficient metal surface protection and high dispersion of the corrosion products. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIII, Issue 4 – 2014 Vira VODYANKA, Sergiy BORUK, Igor WINKLER, New environment friendly etching technologies for some steels and copper alloys, Food and Environment Safety, Volume XIII, Issue 4 – 2014, pag. 376 – 384 384 4. Conclusion An effective and environment friendly composition is proposed for the etching solutions to be used for steel and copper alloys. These compositions ensure signifi- cant reduction in the harmful gases emis- sion and decrease amount of the technolog- ical waste water formation. Specific emis- sion of nitrogen dioxide can be nine times less and formation of the spent solution – four time less comparing to the standard etching compositions. Besides. no special surface cleaning reagents are required in the proposed compositions because of the enhanced surface activity of the compo- nents used. 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