DE10021915C2 - Inhibition of pitting and crevice corrosion - Google Patents

Description

The invention relates to the use of a corrosion present in aqueous solution inhibitors selected from molybdenum compounds, phosphonocarboxylic acids and poly carboxylic acids to inhibit pitting and / or crevice corrosion of stainless steel, nickel and nickel alloys.  

Corrosion means the reaction of a metallic material its environment, which causes a measurable change in the material an impairment of the function of a metallic component or whole system. In most cases, this is reaction electrochemical in nature.

Corrosion phenomena on metals in aqueous solutions mostly run as uniform surface corrosion or as selective corrosion. Of the Selective types of corrosion play pitting in technical applications play an overriding role, whereby these are predominantly based on austenitic CrNi Steels, but also on Ni-based alloys, aluminum or copper materials is observed. Pitting corrosion is particularly triggered in the Media contain ions of the halogens Cl, Br and I.

Methods of preventing corrosion are numerous in the literature described. In the first place, however, corrosion can be determined by a targeted Material selection, optimal processing of the materials or through targeted Prevent interference in the procedural processes of manufacturing processes. The latter method can change the process parameters (Temperature and pressure of the process, flow velocities, etc.) on the other hand, the chemical composition of the Corrosive substance to be changed. This can be a substitution  of ingredients, changing the pH or reducing the content Pull halides with you.

Another method of avoiding corrosion is the targeted addition of Substances in the corrosion medium that represent the drain of the electrochemical Slow down or completely suppress metal dissolution. These additives are called corrosion inhibitors.

Heating and cooling circuits are a common area of application for inhibitors z. B. in the chemical industry and in power plants Inhibitors mainly to avoid flat or trough-shaped Material removal on unalloyed and low-alloy steels as well as on copper and Copper alloys used.

Only a few technologies have become established for the corrosion protection of steel in water-bearing systems that cannot be operated without O ₂ .

An inhibitor gate system on the market includes cathodic protection by zinc salt (zinc (II) ions) in combination with anodic inhibitors based Organophosphate, phosphono- and phosphino-carboxylates, salts of Polycarboxylic acids, phosphate esters or inorganic phosphates. As a disadvantage This method is known to have significant zinc deposits in cooling circuits, which ensures compliance with the German limit value for zinc in waste water of 4 mg / l becomes difficult. Newer inhibitor systems with polycarboxylic acids and Phosphino / phosphono copolymers come with lower ones Zinc concentrations off, making compliance with the zinc limit easier is to be realized.

An alternative to zinc-containing combinations is provided by orthophosphates, which are in more recently through organic phosphates (e.g. phosphonates and phosphate esters) have been replaced. However, phosphates also contribute to wastewater pollution, which limits their use.  

In addition to amines and alkylammonium compounds, they contain molybdate Inhibitors have increased in importance in recent years. Molybdate apply in the usual application concentrations as ecologically harmless. Concentrations of 0.05 to 0.2 g / l molybdenum in the form of molybdate achieve a good corrosion-inhibiting effect.

All of these corrosion inhibitors described act only on the Avoid extensive or trough-shaped material removal. Commercial inhibitors to prevent halide-induced So far, pitting corrosion has not been available. It is known that for example sulfate ions in aqueous media the pitting corrosion behavior austenitic CrNi steels improve, however this finding has so far been not used industrially. The same applies to the positive influence of phosphates (e.g. sodium phosphate), which also increases the pitting corrosion behavior of the CrNi Improve steels.

The invention is based on the knowledge that certain commercially available Corrosion inhibitors such as inhibitors containing molybdate, as described in Cooling circuits to avoid abrasive corrosion on un- and low-alloy steels or copper materials are used, also that Prevent pitting on austenitic CrNi steels. This new one Knowledge was gained on the one hand using a conventional cooling water (ice water), on the other hand using Düsseldorf tap water (Cl content approx. 125-150 mg / l) on material samples made of steel of type 1.4301.  

W. Paatsch reports in "Materials and Corrosion", Vol. 35 (1984), pages 179 to 185 on "Inhibition of pitting corrosion of iron materials in neutral solutions by organic phosphonic acid derivatives ". The inhibitory effect of phosphonic acids, Benzene-1,4-diphosphonic acid, propane-1,3-diphosphonic acid, hexane-1,6-diphosphonic acid, Amino-tris (methylenephosphonic acid) and 1-hydroxyethane-1,1-diphosphonic acid Pure magnetic iron, unalloyed steel, nickel and austenitic steel examined.

US-A-4 717 495 relates to cooling water additive formulations for diesel engines. This Formulations contain the following components: a) Borate and phosphate salts as Buffer, b) nitrite and molybdate salts as pitting inhibitor (cavitation liner pitting inhibitor), c) nitrates, silicates and azole compounds as metal corrosion inhibitors, d) silicones, alcohols hole and glycols as defoamers and e) phosphonates, polyacrylates and sulfonate ver bindings to prevent the formation of deposits.

US-A-5 227 133 describes a method for inhibiting pitting corrosion of Carbon steel in cooling tower systems. Formulations, containing water-soluble molybdates and orthophosphates added.

In contrast, the present invention has the task of using Corrosion inhibitors also in water containing halide, especially in water containing chloride Pitting and / or crevice corrosion of austenitic steel, nickel and nickel alloys prevent.

Accordingly, the present invention relates to the use of at least one in aqueous solution of corrosion inhibitor selected from molybdenum compounds, phosphonocarboxylic acids and polycarboxylic acids for inhibiting pitting and / or crevice corrosion of stainless steel, nickel and nickel alloys.  

The stainless steel can for example be selected from ferritic or martensitic Cr steels, especially those with Cr contents <12% by weight, and made of austenitic CrNi steels with and without Mo or with and without Si, especially those with Cr contents between 16.5% and 30% by weight and Ni contents between 4% and 34% by weight. As nickel can for example pure nickel of the type 2.4066, 2.4068 or 2.4061 are present. The Nickel alloy can be a Ni-based alloy with Ni contents between 36 wt .-% and 72% by weight. It can also contain Cr contents between 0 and 30% by weight and / or Mo contents between 0 and 30% by weight and / or Cu contents between Have 0 and 34% by weight, a total of 100% by weight of course, must not be exceeded.

The polycarboxylic acids can for example be selected from partially saponified Polyacrylamides, polymers or copolymers of acrylic acid and / or Methacrylic acid and from polyaminocarboxylic acids such as Polyaspartic.

The use according to the invention makes it possible to punch and / or Prevent crevice corrosion even in water, if suitably chosen Corrosion inhibitor concentration more than 500 ppm chloride ions, even contain more than 2000 ppm chloride ions, yes up to 5000 ppm chloride ions can.

The pH range of the aqueous solution by the invention Use against pitting and / or crevice corrosion can be inhibited include the wide range of about 1.5 to about 13. Accordingly, the aqueous solution be acidic and for example a pH in the range of about 1.5 to about 5. Examples of such acidic solutions are acidic Cleanser. However, the aqueous solution can also have a pH in the neutral range, thus have in the range from about 5 to about 8. Examples of such waters are Urban or industrial water or sea water. Or the pH is in the  weakly basic area, for example in the range from about 8 to about 10. Auf this pH range is often the cooling water in cooling circuits from Industrial companies and power plants discontinued.

The aqueous solution preferably contains the corrosion inhibitor (s) in following concentration ranges: molybdenum compounds in an amount that 0.04 to 1 g / l corresponds to molybdenum, 0.1 to 1 g / l Phosphonocarboxylic acids, 0.02 to 1 g / l polycarboxylic acids.

In a preferred embodiment, the aqueous solution contains at least a corrosion inhibitor selected from compounds of molybdenum in the Oxidation level +6. Examples include molybdic acid and molybdates, for example in the form of sodium, potassium and / or ammonium salts. there it may be advantageous if the aqueous solution in addition to Molybdenum compound selected from at least one corrosion inhibitor partially saponified polyacrylamides, polymers or copolymers of acrylic acid and / or methacrylic acid and polyaminocarboxylic acids, in particular Homopolymers or copolymers of acrylic acid and / or methacrylic acid.

A suitable phosphonocarboxylic acid is, for example Phosphono-1,2,4-tricarboxylic acid.

The aqueous solution can be, for example, the circulating water in an open or a closed cooling circuit. Such Cooling circuits are mainly found in steelworks, power plants and in the chemical and especially in the petrochemical industry. Here is the Use in closed cooling circuits preferred, because it is lighter can be ensured that the inhibited aqueous solution is not without previous water treatment is released into the environment.

However, the aqueous solution can also be cooling water act in an open cooling circuit. Here, the invention  Use the particular advantage that when concentrating the Cooling water can be tolerated by evaporation higher chloride concentrations can than usual. As a result, the time intervals between the required replacement or supplemental intervals of the cooling water extended become.

Another embodiment of the use according to the invention is that the aqueous solution is an acidic (pH in the range of about 1.5 to about 5) or alkaline (pH about 8 to about 13) cleaning and / or disinfectant solution. Such cleaning and / or Disinfectant solutions can be found on very different technical Areas are used. The cleaning of Reaction vessels or other containers as well as pipelines in the chemical industry as well as in the food and pharmaceutical industries. by virtue of The corrosion inhibition can be the acidic or alkaline cleaning and / or Disinfectant solution longer in contact with the surfaces to be cleaned remain as usual. Because of the better cleaning achieved in this way and / or disinfection, the intervals between two cleaning and / or Disinfection cycles can be extended. This is particularly advantageous in the Food and pharmaceutical industry, because of the longer exposure to the Disinfection effect intensified and thus the production security is increased. at Maintaining the previous exposure time will extend the lifespan of the Items increased.

A specific example of use in the food industry is that the aqueous solution is a solution for cleaning and Disinfect bottling and bottle cleaning systems. Here it can according to the current state of the art, especially in peripheral areas outside of Spray zone come to punctiform corrosion phenomena, of which can result in drops of the cleaning solution evaporating Concentrate and thereby lead to selective increased corrosion. This effect can at least be achieved through the use according to the invention be restricted.  

A specific example of the use according to the invention is that the Cleaning and / or disinfectant solution is acidic and oxidizing agent, selected from active chlorine, chlorine dioxide and peroxo compounds. Active chlorine can be aqueous solutions of chlorine. It can however, they are also compounds dissolved in the aqueous solution which Release chlorine spontaneously or in contact with air or oxidizable substances. Examples include hypochlorite or chloroisocyanuric acid. Both Peroxo compounds can be, for example, hydrogen peroxide, the Salts or act as hydrogen peroxide-forming compounds. Farther come as peroxo compounds peracids like peroxosulfuric acid or Peroxodisulfuric acid into consideration. In particular, the peracids Percarboxylic acids such as performic acid or peracetic acid represent. In addition to peracetic acid, mono- or di-peracids also come with one longer carbon chain, for example with 3 to 12 carbon atoms and their Esters into consideration. Such solutions have due to the oxidizing effect in addition to the cleaning, a pronounced disinfection effect. An important Such solutions are used in the food industry such as for example in the beverage and milk industries.

The inhibited aqueous solution can also be in the form of a foam with the mentioned surfaces are brought into contact. An area of application acidic or alkaline foam cleaners, which have the advantage, are suitable for this sloping or vertical surfaces long enough to hold you to cause sufficient cleaning and / or disinfection effect.

In a further embodiment of the use according to the invention the aqueous solution is sea or brackish water. Such water indicates particularly high concentrations of dissolved salts, in particular Table salt on. The average salinity, of which about 80% by weight Chloride is absent, is about 34 to 35 g / l. One area of application is Use according to the invention in that such saline sea or Brackish water can be used for cooling purposes without a perforated  and / or crevice corrosion is to be feared. Another area of application is in the desalination of sea or brackish water for drinking or Industrial water.

The advantage of the use according to the invention is therefore generally that that the vulnerability of stainless steel, nickel and nickel alloys to perforated and / or crevice corrosion is reduced when this material is in contact with Water and especially with water containing chloride. With the same Steel type can extend the exposure time of the aqueous solution before the corrosion phenomena mentioned occur. This is interesting for cleaning and / or disinfection processes, for example Effectiveness can be increased by a longer period of time. Or it can a higher concentration due to the use according to the invention Chloride ions are tolerated than previously common. This can be for cooling circuits be advantageous, which can be operated with more saline water. As a result, more saline water can be used for cooling purposes and / or the period between flooding measures can be extended. Generally it is possible due to the use according to the invention, higher salinity Use water as usual for various technical purposes. For example, plants in which previously completely demineralized water can be made Corrosion protection had to be used due to the invention normal inhibited well or drinking water.

If you accept the same corrosion as in the prior art, it is possible due to the use according to the invention of lower alloys and thus to use cheaper materials in the systems concerned than previously common. For example, instead of the high-alloy steel of the type 1.4401 low alloy chrome-nickel steel, for example of the type 1.4301 can be used because the inhibition according to the invention Pitting corrosion risk is reduced.

embodiments

In a NaCl-containing aqueous test solution (H ₂ O distilled + 2000 mg / l chloride), the pitting corrosion behavior of steel 1.4301 was examined at room temperature, whereby the individual corrosion inhibitors or mixtures thereof were systematically added to this basic solution. The tendency of the substrates to corrode was assessed by measuring the electrochemical pitting corrosion potentials and their difference from the resting potentials. The results of this series of experiments are shown in Table 1. Since in practice, in addition to the pitting corrosion potential, the difference between pitting and resting potential is used to estimate the risk of pitting, the addition of molybdate and polyacrylate (Sokalan® CP10, BASF) results in good inhibition in the base solution. This is documented by the comparatively high values for ΔU. In practice, it is assumed that the ΔU value must be <250 mV to ensure adequate pitting corrosion resistance. While the basic solution does not have sufficient pitting corrosion resistance in steel 1.4301 (ΔU = 125 mV <250 mV), this limit is exceeded by adding molybdate (ΔU = 640 mV) and sodium polyacrylate (ΔU = 380 mV). The substance mixture "mixture 1" also has a good inhibitory effect against pitting corrosion (ΔU = 480 mV) in this basic solution. It remains to be noted, however, that in these experiments the pH value was changed compared to the base solution by adding the various chemicals, as a rule it was increased, so that an influence on the test result by this parameter cannot be excluded. In cooling systems, however, changes in pH can be accepted in many cases, so that the above results can be regarded as representative for this area of application.

In a further series of tests, it was tested whether pitting corrosion on CrNi steels can be prevented by selected inhibitors even under acidic attack conditions. The NaCl-containing test solution (H ₂ O distilled + 2000 mg / l chloride), which was adjusted to a pH of approximately 2.0 with sulfuric acid, was used as the basic solution. The pH values of the solutions formed by adding the inhibitor solutions were also adjusted to approximately 2 by metering in H ₂ SO ₄ . Only when using the acid phosphonobutane-1,2,4-tricarboxylic acid was it possible to dispense with sulfuric acid. The results of the tests carried out under acidic attack conditions and constant pH are shown in Table 2.

As expected, the pitting corrosion resistance of steel 1.4301 deteriorates considerably due to the acidic attack conditions. There is an acute risk of pitting in the acidic base solution (without inhibitor), since the pitting corrosion potential is lower than the resting potential. However, by adding mixture 1 or selected inhibitors, an inhibition of the Cl ^- ion attack is also found under acidic attack conditions. The determined ΔU values are, without exception, higher than in the inhibitor-free basic solution. When adding 2 ml / l alkaline molybdate solution, a value of ΔU = 250 mV is clearly exceeded, which indicates a particularly effective inhibition of pitting corrosion.

Test results appear particularly interesting, according to which this inhibitor and mixtures containing it also prevent pitting corrosion on steels of type 1.4301 in acidic disinfectant solutions for the beverage and cosmetics industries. In a first test series, by adding 2 ml of mixture 1 per liter of a 2%, 40 ° C warm application solution of the cleaning and disinfectant "mixture 2", material 1.4301 succeeded against the attack of 500 mg / l Cl ^- - To protect ions sufficiently, ie pitting corrosion could be prevented. Appropriate measurements have shown that the necessary consistency of the disinfectant peracetic acid is not impaired by the addition of the inhibitor.

In long-term exposure tests carried out on a random basis, the dosing of the inhibitor to avoid crevice corrosion was also checked without adjusting the pH. For this purpose, specimens made of steel 1.4301 were immersed in the solution in a 2% working solution of mixture 2 with a Cl ^- content of 500 mg / l at 40 ° C over a period of 1 week. While both hole corrosion on the sample surface and corrosion under the gap body were found in the untreated application solution, the sample that was stored in a solution treated with 2 ml / l alkaline molybdate solution (inhibitor solution 1) showed no pitting corrosion and one under the gap body the inhibitor-free solution significantly reduced crevice corrosion. If the Cl ^- concentration in this experiment is only 250 mg / l, pitting and crevice corrosion phenomena occur again in the uninhibited 2% solution of mixture 2. The samples, in which 2 ml / l mixture 1 or alkaline molybdate solution were added, were completely corrosion-free.

The mixtures and used above and in the tables below Solutions have the following composition (in parts by weight):

Mix 1

22 phosphonobutane-1,2,4-tricarboxylic acid, 50%
5 polyacrylate solution, 45%
5 triethanolamine
11 sodium molybdate dihydrate
39 water
pH adjusted so that a 1% solution has a pH of 10.9.

Mix 2

9.8 peracetic acid
11.8 hydrogen peroxide
18.8 acetic acid
3.75 phosphoric acid
8 sulfuric acid
47.85 water

Inhibitor solution 1 (solution 1): potassium molybdate solution with 13.6% by weight Mo and 17.8 up to 18.4% by weight KOH

Inhibitor solution 2 (solution 2): polyacrylate solution, 45%

Inhibitor solution 3 (solution 3): phosphonobutane-1,2,4-tricarboxylic acid solution, 50%  

Table 1

Results of electrochemical pitting corrosion tests (steel 1.4301)

Table 2

Results of electrochemical pitting corrosion tests (steel 1.4301) in an acidic environment (adjustment of the pH to 2 using sulfuric acid)

Claims (12)

1. Use of at least one present in aqueous solution Corrosion inhibitor selected from molybdenum compounds, Phosphonocarboxylic acids and polycarboxylic acids for inhibiting pitting and / or crevice corrosion of stainless steel, nickel and nickel alloys. 2. Use according to claim 1, characterized in that the steel is selected from ferritic or martensitic Cr steels and from austenitic CrNi steels. 3. Use according to claim 1, characterized in that the Nickel alloy a Ni-based alloy with Ni contents between 36% by weight and represents 72% by weight. 4. Use according to one or more of claims 1 to 3, characterized characterized in that the polycarboxylic acids are selected from partially saponified Polyacrylamides, polymers or copolymers of acrylic acid and / or Methacrylic acid and from polyaminocarboxylic acids. 5. Use according to one or more of claims 1 to 4 thereby characterized in that the aqueous solution or the corrosion inhibitors in contains the following concentration ranges: Molybdenum compounds in one Amount corresponding to 0.04 to 1 g / l molybdenum, 0.1 to 1 g / l Phosphonocarboxylic acids, 0.02 to 1 g / l polycarboxylic acids. 6. Use according to one or more of claims 1 to 5, characterized characterized in that the aqueous solution has at least one corrosion inhibitor selected from compounds of molybdenum in the oxidation state +6 contains. 7. Use according to claim 6, characterized in that the aqueous Solution additionally selected from at least one corrosion inhibitor  partially saponified polyacrylamides, polymers or copolymers of acrylic acid and / or methacrylic acid and from polyaminocarboxylic acids. 8. Use according to one or more of claims 1 to 7, characterized characterized in that the aqueous solution is an acidic or alkaline cleaning and / or disinfectant solution. 9. Use according to claim 8, characterized in that the cleaning and / or disinfectant solution is acidic and selected oxidizing agents contains active chlorine, chlorine dioxide and peroxo compounds. 10. Use according to one or more of claims 1 to 9, characterized characterized in that the aqueous solution in the form of a foam with the Stainless steel, the nickel and / or the nickel alloys brought into contact becomes. 11. Use according to one or more of claims 1 to 7, characterized characterized in that it is cooling water in the aqueous solution. 12. Use according to one or more of claims 1 to 7, characterized characterized in that the aqueous solution is sea or Brackish water.