DE102014003770B4 - Process for inhibiting corrosion of water-conducting systems - Google Patents

Abstract

Process for inhibiting the corrosion of water-bearing systems made of low-alloy or unalloyed steel, of copper or copper-containing alloys or of lead, characterized in that the water-bearing system is supplied with a thermally activated, aqueous solution of alkali silicate of the formula M2O · n SiO2 with n = 1 to 4 andM = Na and/or potassium is added as a corrosion inhibitor, the alkali silicate solution being activated by heating to temperatures in the range from 60 to 150 ° C, and the alkali silicate solution being added in such an amount that the water in the system is silicate in a concentration in the range of 1 to 100 mg/l, calculated as SiO2.

Description

The present invention relates to a method for inhibiting corrosion of water-conducting systems which consist of low-alloy or unalloyed steel, copper or copper-containing alloys or lead, in which thermally activated silicates are metered into the water systems.

In the context of the invention, water-carrying systems are to be understood as meaning, for example, drinking water pipes or process or cooling water pipes, in particular drinking water distribution networks, domestic installations, cooling circuits, in particular those that are operated in a continuous or drain manner, sewage pipes, process water systems in which water is led through pipes, heat exchangers, storage containers and treatment systems becomes. In water-conducting systems, the water comes into contact with metal surfaces made, for example, of unalloyed or alloyed steel, copper or copper-containing alloys, aluminum or galvanized steel. The invention specifically relates to water-bearing systems that contain copper or copper-containing materials.

From the EP 0 860 517 B1 It is known to add certain chemical substances to the water in water-bearing metal systems to prevent corrosion and the associated impairment of water quality. For flow systems, especially for drinking water supply, only a few substances come into question because the health risk to consumers must be avoided and pollution of the wastewater must be avoided as far as possible.

The use of silicates is known to inhibit corrosion of metal systems carrying drinking water and is described, for example, in the Technical Rule DVGW W 215-2:2010-04, Central use of corrosion inhibitors - Part 2: Silicate mixtures. The silicate is usually used in combination with phosphates, particularly to prevent copper corrosion.

The silicate mixtures according to bbr 04/2010,60-65 are generally used as a liquid product. In contrast to the central dosing of phosphates, powdered solid products play no role here. Silicates and silicate mixtures are delivered as ready-made dosing solutions and stored in suitable facilities, such as storage tanks, from which they are usually dosed into the drinking water using dosing pumps. Storage and dosing takes place below ambient temperature. In some cases the dosing line is heated to approx. 25°C to prevent crystallization in the dosing line.

Pure silicates are generally not used, but rather mixtures, especially with phosphates, since silicates have a lower specific effect compared to phosphates, especially against copper corrosion. Silicates are also used in mixtures with carbonates and alkali metal hydroxides.

DE 197 55 622 A1 describes a stable aqueous concentrate containing silicate and phosphate ions in a ratio of 1:2 to 1:4 with a total phosphate content ≥ 6% and ≥ 1.5% SiO ₂ for corrosion protection of drinking water pipes.

DE 43 21 883 A1 discloses a stable aqueous concentrate for the same use containing at least 8 wt% silicate (as SiO ₂ ) and at least 3 wt% orthophosphate (as PO ₄ ).

EP 0 860 517 B1 describes a method for anti-corrosion treatment of water-bearing metal systems by metering a combination of phosphates and silicates into the water stream, characterized in that phosphates and silicates are metered separately from one another.

DE 197 06 410 A1 describes a process for the corrosion protection treatment of water-bearing metal systems by metering a combination of phosphates and silicates into the water stream, characterized in that phosphates and silicates are metered in separately from one another. It also describes DE 197 06 410 A1 Means of carrying out the procedure.

EP 1 780 310 B1 claims a method for corrosion protection treatment of water-bearing metal systems by metering a combination of silicates and phosphates into the water stream of the water-bearing metal system, characterized in that it comprises the steps: mixing a first solution containing the phosphates with a second solution containing the silicates , to a third solution, whereby a ripening process that can be recognized by an increase in turbidity and/or viscosity occurs in the third solution, and - interrupting the ripening process by feeding the third solution into the Main water flow of the water-bearing metal system after either the turbidity relative to the turbidity immediately after combining the two solutions by a factor in the range of 1.5 to 50 or the viscosity of the solution relative to the viscosity immediately after combining the two solutions by a factor of 1 .5 to 10 has increased.

The disadvantage of all the mixtures and processes described is that the corrosion inhibitor contains phosphate and this leads to phosphate contamination of the wastewater. Phosphate promotes the eutrophication of water, so there is a need for a phosphate-free corrosion inhibitor that has improved corrosion inhibition compared to silicates without phosphate.

Surprisingly, it has now been found that the anti-corrosive effectiveness of alkali silicate solutions can be significantly increased if the alkali silicate solution is thermally activated shortly before use in the water-bearing system. It was not possible to determine exactly which physicochemical processes take place during thermal activation.

The object of the invention is achieved by providing a method for inhibiting the corrosion of water-bearing systems made of low-alloy or unalloyed steel, of copper or copper-containing alloys or of lead, the method being characterized in that the water-bearing system is supplied with a thermally activated, aqueous solution of alkali silicate of the formula M ₂ O · n SiO ₂ with n = 1 to 4 and M = Na and / or potassium is metered in as a corrosion inhibitor, the alkali silicate solution being activated by heating to temperatures in the range from 60 to 150 ° C, and where the alkali silicate solution is added in such an amount that the water in the system contains silicate in a concentration in the range of 1 to 100 mg/l, calculated as SiO ₂ .

Water-soluble sodium silicate, potassium silicate or a mixed sodium-potassium silicate and also mixtures of these can be used as the alkali metal silicate. Foreign ions/impurities commonly found in technical products do not cause any problems, but with the restriction that they must not lead to limit values being exceeded when treating drinking water systems.

According to the invention, a commercially available water glass solution can in particular be used as an alkali silicate solution. Water glass solutions are created by dissolving solid water glasses in water. It is known that liquid water glass solutions are colloids with high viscosity and surface tension, which form a gelatinous form and hardly crystallize. Their pH value in aqueous solution is 12, because the salts of the weakly acidic silica release OH ^- - ions during hydrolysis, according to the equation: Me ₄ SiO ₄ + 3 H ₂ O <--> 4 Me+ 3 OH- + H ₃ SiO ₄ It is also known that at higher temperatures, hydrolytic cleavage and alkalinity increase and viscosity decreases.

The alkali silicate solution usually contains from 10 to 40% by weight, preferably from 15 to 35% by weight, of silicate, calculated as SiO ₂ . Typically, the alkali silicate solution contains from 4 to 25% by weight, preferably from 7 to 20% by weight, of alkali, calculated as alkali metal oxide.

The thermal activation preferably takes place at temperatures between 60 and 130 ° C, particularly preferably between 70 and 130 ° C, in particular between 80 and 115 ° C, preferably over a period of 0.5 to 170 hours, particularly preferably 3 to 70 hours .

In a preferred embodiment, the activation takes place in two or more stages by bringing the alkali metal silicate solution to a first temperature, in particular a lower temperature of preferably 70 to 100 ° C, for a first period of time, in particular a longer period of time of preferably 20 to 50 hours, and then for a second period of time, in particular a shorter period of time of preferably 0.5 to 5 hours, to a second temperature, preferably a higher temperature of preferably 100 to 130 ° C.

If the thermal activation takes place at temperatures above the boiling point of the water in the solution, it takes place in a closed, pressure-tight container, for example an autoclave.

The thermal activation is reversible, so that activation preferably takes place near the dosing point. After thermal activation, the dosage into the drinking water usually takes place within 24 hours, preferably within 10 hours, particularly preferably within 4 hours, especially within 1 hour.

According to the invention, the thermally activated alkali silicate solution is added in such an amount that the water in the water-conducting system contains silicate in a concentration in the range from 1 to 100 mg/l, preferably from 2 to 20 mg/l, calculated as SiO ₂ . Particularly preferably, the alkali silicate solution is diluted with heated and/or fully demineralized water, in particular with heated, fully demineralized water, before adding. The temperature of the heated water is preferably in the range of 60 to 90 °C.

Further corrosion inhibitors, preferably phosphates, in particular alkali metal phosphates, and/or carbonates, in particular alkali metal carbonates, and/or hydroxides, in particular alkali metal hydroxides, can also be added. The other corrosion inhibitors can be added to the alkali silicate solution before or after thermal activation.

The thermal activation can also be carried out with mixtures of an alkali metal silicate solution with phosphates and/or carbonates and/or alkali metal hydroxides. Likewise, the thermally activated alkali metal silicate can be used in the processes according to the teachings of EP 0 860 517 B1 and EP 1 780 310 B1 be used.

However, the thermally activated alkali metal silicate solution according to the invention shows good effectiveness even without further corrosion inhibitors and is therefore preferably used without these additives, particularly in drinking water systems.

The invention will be explained using the following examples, but without being limited to the specifically described embodiments. Unless otherwise stated or the context dictates otherwise, percentages refer to the weight, or in case of doubt to the total weight of the mixture.

The invention also relates to all combinations of preferred embodiments, as long as they are not mutually exclusive. The statements “about” or “approximately” in conjunction with a numerical indication mean that values that are at least 10% higher or lower, or values that are 5% higher or lower, and in any case 1% higher or lower are included.

Test method for determining corrosion

1 liter of the test water (see Table 1 for composition) is placed in a beaker. Three copper coupons (CDA 110) are attached to a holder and rotated vertically using a stirring motor in the test water at 50 revolutions/minute for a period of 24 hours. The coupons are completely immersed in the test water. The test water is discarded and the beaker is filled again with 1 liter of freshly prepared test water of the same composition, the concentration of silicate to be tested is added and stirred at room temperature for a period of 168 hours (7 days).

• 2.1. der Anwendungsbereich liegt bei zwischen 0,1 bis 5,0 mg/L;
• 2.2. es wird mit einer Luft-Acetylen-Flamme gearbeitet und die Extinktion wird bei einer Wellenlänge von 324,8 nm gemessen;
• 2.6. die Salpetersäure/Cäsium-Lösung bestehend aus 500 ml Salpetersäure Suprapur, 500 ml vollentsalztem-Wasser und 100 g Cäsiumchlorid p.a. Kupfer-Stammlösung, ß(Cu) = 1000 mg/L Kupfer-Bezugslösung; ß(Cu) = 5 mg/L die Bezugslösung beinhaltet auf 100 ml 1 ml der Salpetersäure/Cäsium - Lösung, als Blindwertlösung/Nullwertlösung dient vollentsalztes Wasser ohne Salpetersäure;
• 2.7. die zu untersuchenden Proben werden in Kunststoffbehältern gelagert und direkt nach der Probenahme mit der Salpetersäure/Cäsium - Lösung angesäuert, wobei auf 10 ml Probe 0,1 ml Säure-Lösung gegeben werden. Die Proben werden direkt gemessen.

Corrosion is determined by measuring the copper concentration after 168 hours in the solution using atomic absorption spectroscopy based on DIN 38 406 - E7 - 1 with the following deviations from the standard:

• 2.1. the application range is between 0.1 and 5.0 mg/L;
• 2.2. an air-acetylene flame is used and the extinction is measured at a wavelength of 324.8 nm;
• 2.6. the nitric acid/cesium solution consisting of 500 ml Suprapur nitric acid, 500 ml deionized water and 100 g cesium chloride pa copper stock solution, ß(Cu) = 1000 mg/L copper reference solution; ß(Cu) = 5 mg/L the reference solution contains 1 ml of the nitric acid/cesium solution for every 100 ml; demineralized water without nitric acid serves as the blank value solution/zero value solution;
• 2.7. The samples to be examined are stored in plastic containers and acidified with the nitric acid/cesium solution immediately after sampling, adding 0.1 ml of acid solution to 10 ml of sample. The samples are measured directly.

The measured copper concentration is a measure of corrosion. Table 1: Composition of test water: parameter Unit Test water 1 Test water 2 Calcium Mol/ ^m3 1.2 2.3 magnesium Mol/ ^m3 0.3 0.5 Hydrogen carbonate Mol/ ^m3 1.9 3.7 sulfate Mol/ ^m3 0.4 0.7 chloride Mol/ ^m3 1.7 3.4 sodium Mol/ ^m3 2.2 4.4 Nitrates Mol/ ^m3 0.8 1.6 pH 8.2 8.5

Thermal activation:

For thermal activation, 200 ml of commercially available water glass (WG1: NaWG 38/40 from BASF; WG2: NaWG 37/40 from Woellner) were filled into a 480 ml Teflon vessel and stored in a drying cabinet at the activation temperature for a specified period of time. Activation at temperatures above 100 ° C was carried out in an autoclave (pressure cooker from Fissler, 10 liters, 115 ° C, 75 kPa operating pressure).

The stock solutions for dosing were prepared with demineralized water, which was also stored in a drying cabinet at the activation temperature or at 90°C in a drying cabinet for thermal activation above 100°C. The thermally activated silicate was dosed from the heated stock solution into the beakers.

Test water without dosage was used as a blank value. Commercially available water glass was used as a reference inhibitor.

Example 1:

Thermal activation took place at 80°C over a period of 1 day. WG1 and test water 1 were used: Concentration WG1 [mg SiO ₂ /L] Thermal activation Copper concentration [mg/L] 0 - 1.00 5 without 0.82 10 without 0.76 15 without 0.69 5 with 0.77 10 with 0.68 15 with 0.52

It can be clearly seen that the copper concentration in the test water after the end of the test is significantly lower with thermally activated silicate than with the same concentration without thermal activation.

Example 2:

WG1 and test water 1 were used. Thermal activation took place at 80°C. Concentration WG1 [mg SiO ₂ /L] Duration of thermal activation [days] Copper concentration [mg/L] 0 - 1.00 15 without 0.69 15 1 0.52 15 2 0.43 15 3 0.43 15 4 0.47

It can be clearly seen that the copper concentration in the test water after the end of the test is significantly lower with thermally activated silicate than with the same concentration without thermal activation. The improvement in corrosion inhibition depends on the duration of thermal activation.

Example 3:

WG1 and test water 1 were used. The thermal activation period was 3 days. Concentration WG1 [mg SiO ₂ /L] Temperature thermal activation Copper concentration [mg/L] 0 - 1.00 15 without 0.69 15 40 0.66 15 60 0.65 15 80 0.43

It can be clearly seen that the copper concentration in the test water after the end of the test is significantly lower with thermally activated silicate than with the same concentration without thermal activation. The improvement in corrosion inhibition depends on the temperature of thermal activation.

Example 4:

WG1 and test water 1 were used. Thermal activations at one and two temperatures were used. Concentration WG1 [mg SiO ₂ /L] Thermal activation Copper concentration [mg/L] 0 - 1.00 15 without 0.69 15 1 hour at 90 °C 0.66 15 2 hours at 115°C 0.61 15 3d at 80°C 0.43 15 3d at 80 °C and 0.37 1 hour at 90 °C

It can be clearly seen that the copper concentration in the test water after the end of the test is significantly lower with thermally activated silicate than with the same concentration without thermal activation. Thermal activation at two temperatures shows significantly better corrosion inhibition than thermal activation at one temperature.

Example 5:

WG1 and both test water 1 and test water 2 were used. Thermal activation took place at 80 °C for 3 days. The stock solution was prepared on the one hand immediately after thermal activation at 80 °C (time difference 0) and, for comparison, after 24 hours and cooling to room temperature (time difference 24 h). In the case of test water 2, the copper concentration was determined after just 6 days of testing. Concentration WG1 [mg SiO ₂ /L] thermal activation Time difference Copper concentration [mg/L] test water 1 Copper concentration [mg/L] test water 2# 0 - - 1.00 0.99 15 Without - 0.69 0.89 15 with 0 0.43 0.64 15 with 24 hours 0.61 0.72 # after 6 days of testing

From Example 5 it can be seen that the thermal activation is reversible. If the thermally activated silicate is cooled to room temperature and stored for 24 hours and the stock solution is created from it, the copper concentration is higher, so the corrosion inhibition is worse compared to the experiment in which the stock solution was created with 80 ° C warm water immediately after the end of thermal activation is dosed immediately afterwards. However, there is still an improvement compared to the non-activated solution.

Example 6:

Thermal activation took place at 115 °C for 2 hours. Test water 2 was used. Concentration [mg SiO ₂ /L] thermal activation Water glass Copper concentration [mg/L] 0 - - 0.99 15 without WG1 0.89 15 without WG2 0.88 15 with WG1 0.74# 15 with WG2 0.73# # after 8 days of testing

From Example 6 it can be seen that thermal activation leads to the same improvement in corrosion inhibition for both commercially available water glasses.

Claims (18)

Process for inhibiting the corrosion of water-bearing systems made of low-alloy or unalloyed steel, of copper or copper-containing alloys or of lead, characterized in that the water-bearing system is supplied with a thermally activated, aqueous solution of alkali silicate of the formula M ₂ O · n SiO ₂ with n = 1 to 4 and M = Na and / or potassium is added as a corrosion inhibitor, the alkali silicate solution being activated by heating to temperatures in the range from 60 to 150 ° C, and the alkali silicate solution being added in such an amount that the water in the system contains silicate in a concentration in the range of 1 to 100 mg/l, calculated as SiO ₂ . Procedure according to Claim 1 that the alkali silicate solution is activated by heating to temperatures in the range from 70 to 130 ° C, in particular from 80 to 115 ° C. Procedure according to Claim 2 , characterized in that the alkali silicate solution is activated over a period of 0.5 to 170 hours, preferably 3 to 100 hours, particularly preferably 10 to 50 hours. Procedure according to one of the Claims 1 until 3 , characterized in that the activation takes place in two or more stages by bringing the alkali silicate solution to a first temperature, in particular from 70 to 100 ° C, for a first period of time, in particular from 20 to 50 hours, and then for a second period of time, in particular from 0.5 to 5 hours, to a second temperature, preferably from 100 to 130 ° C. Procedure according to one of the Claims 1 until 4 , characterized in that the alkali silicate solution contains from 10 to 40% by weight, preferably from 15 to 35% by weight, of silicate, calculated as SiO ₂ . Procedure according to one of the Claims 1 until 5 , characterized in that the alkali silicate solution contains from 4 to 25% by weight, preferably from 7 to 20% by weight, of alkali, calculated as alkali metal oxide. Procedure according to one of the Claims 1 until 6 , characterized in that further corrosion inhibitors, preferably phosphates, in particular alkali metal phosphates, and/or carbonates, in particular alkali metal carbonates, and/or hydroxides, in particular alkali metal hydroxides, are added. Procedure according to Claim 7 , characterized in that the further corrosion inhibitors are added to the alkali silicate solution before or after thermal activation. Procedure according to one of the Claims 1 until 8th , characterized in that the alkali silicate solution is added in such an amount that the water in the system contains silicate in a concentration in the range of 2 to 20 mg/l, calculated as SiO ₂ . Procedure according to one of the Claims 1 until 9 , characterized in that the alkali silicate solution is diluted before adding with heated water, in particular fully desalinated water, preferably with water heated to 60 to 90 ° C, in particular fully desalinated water. Procedure according to one of the Claims 1 until 10 , characterized in that the alkali silicate solution is added to the water within a maximum of 24 hours, preferably within a maximum of 10 hours, particularly preferably within 4 hours, in particular within 1 hour, after activation. Process for activating an alkali silicate solution for use as a corrosion inhibitor in water-conducting systems, characterized in that the alkali silicate solution is heated to temperatures in the range from 60 to 100 hours, preferably from 3 to 100 hours, in particular 10 to 50 hours 130 ° C, preferably from 70 to 110 ° C, in particular from 80 to 100 ° C. Procedure according to Claim 12 , characterized in that the alkali silicate solution contains from 10 to 40% by weight, preferably from 15 to 35% by weight, of silicate, calculated as SiO ₂ . Procedure according to Claim 11 or 13 , characterized in that the alkali silicate solution contains from 4 to 25% by weight, preferably from 7 to 20% by weight, of alkali metal oxide. Thermally activated alkali silicate solution available according to one of the Claims 12 until 14 , whereby an aqueous solution of alkali silicate of the formula M ₂ O · n SiO ₂ with n = 1 to 4 and M = Na and / or potassium is used for activation. Use of a thermally activated alkali silicate solution for inhibiting corrosion in water-conducting systems made of low-alloy or unalloyed steel, of copper or copper-containing alloys or of lead, characterized in that according to one of the Claims 1 until 11 an alkali silicate solution is added to the water. Use according to Claim 16 , characterized in that the water-carrying system is made of copper or a copper-containing alloy, in particular copper. Use according to Claim 16 or 17 , characterized in that the thermally activated alkali silicate solution is added as the only corrosion inhibitor.