WO2010108517A1

WO2010108517A1 - Electropolishing method and electromagnetic flowmeter having electropolished electrodes

Info

Publication number: WO2010108517A1
Application number: PCT/EP2009/002248
Authority: WO
Inventors: Hans Jørgen GABEL; Per MØLLER
Original assignee: Siemens Aktiengesellschaft
Priority date: 2009-03-26
Filing date: 2009-03-26
Publication date: 2010-09-30

Abstract

The present invention provides a method for polishing a surface (17) of an electrode (16) composed of at least a first metal. The method comprises immersing said surface (17) of said electrode (16) in an electrolytic solution (42) and applying a positive DC voltage to said electrode. The positive DC voltage is applied for a controlled time interval (ti) to generate a peak current density (56) at said surface (17) that causes formation of at least one reactive oxygen species (ROS) in said solution (42). The ROS is reactive to dissolve said first metal from micro-peaks (44) at said surface (17) of said electrode (16) to smoothen said surface (17). The present invention further provides an electromagnetic flowmeter (10) having measuring electrodes (16,18) having surfaces that are polished by the above method.

Description

Electropolishing method and electromagnetic flowmeter having electropolished electrodes

The present invention relates to electropolishing, in particular, to electropolishing of measuring electrodes of an electromagnetic flowmeter.

Electromagnetic flowmeters utilize the principle of electrodynamic induction for flow rate measurement of a fluid medium. In an electromagnetic flowmeter, a magnetic field is generated across a measuring section of the flowmeter pipe through which the medium flows, which, by operation of Faraday's law, generates a voltage perpendicular to both the flow of the medium and the magnetic field. The induced voltage is measured by a pair of electrodes on opposite sides of the measuring section. This induced voltage measured by these electrodes is proportional to the flow velocity of the medium to be measured averaged over the cross-section of the pipe.

The measuring electrodes, being metallic, are prone corrosion effects due to contact with the fluid. The problem has until now been solved in choosing the right type of material used as electrode material, for example, replacing a commonly used stainless steel electrode with a more corrosion resistant electrode made of Hastelloy™. Other types of electrode materials could also be chosen (for example, Monel™, Titanium, Tantalum, Platinum etc.) depending on the fluid media. The chosen electrode material would insure a high resistance towards corrosion in the given fluid media.

Corrosion at the surface of the measuring electrodes further produces flow noise at different places in the measuring chain. The term ^Λflow noise' or ^λflow measurement noise' is used to indicate any noise that superimposes on the induced voltage in the measuring electrodes. For example, in this context, such flow noise may be triggered by electro-chemical reactions/electron transfer between the electrodes and the media. In particular, under low power consumption (which is often an operational requirement) , an electromagnetic flowmeter becomes increasingly sensitive to such noise phenomena, resulting in a low signal-to-noise ratio.

It is commonly known in corrosion science that a perfectly smooth and homogenous surface is more resistant to corrosion, than a non-homogeneous surface with many defects. Smoothness and homogeneity of the surface of the measuring electrodes can be achieved by conventional electropolishing. The process contains two steps, namely, a cathodic degreasing procedure of the surface based on an alkaline solution (NaOH and KOH) , followed by an acidic electrolyte (mixture of concentrated sulfuric acid and concentrated phosphoric acid) , wherein the electrode to be polished is connected as the anode to a DC power supply. Usually the acid bath is held at a constant temperature, for example at 6O⁰C, and the involved current densities are typically around 35 Amp/dm2. The area used to calculate the current is the area exposed to the acid. For example, in the case of stainless steel electrodes, gradual application of a DC voltage leads to the formation of a film comprising a complex chromium sulfate (Cr2 (SO₄) ₃ *14H₂O) and possibly also chromium hydroxide (Cr(OH)₃) at the surface of the electrode. This film controls the smoothening of the surface in conventional electropolishing. On increasing the voltage of the DC power supply, this film starts dissolving. The dissolution of the film takes place at the "micro-peaks", which are more exposed to the acidic electrolyte than the "micro-valleys". In short, the principle in electro polishing of a surface is that a coating consisting of oxides or salts protects the micro-valleys from dissolution, while the micro- peaks are attacked. The effect of this is that the surface is smoothened.

However there are certain disadvantages with conventional electropolishing. For example, in conventional electropolishing, as the voltage is increased, it may lead to the formation of oxygen gas at the anode which may disturb the uniformity of the film. Problems caused by oxygen formation will in practice show itself as areas, where the polishing is unequal due to partial destruction of the above- mentioned film. Furthermore, it is very often impossible to apply high current density on stainless steel components having large surface area, because the material does not allow conduction of the current caused by a limited cross section in the component to be polished.

The object of the present invention is to provide an improved method of electropolishing. The above object is achieved by a method according to claim 1.

It is a further object of the present invention to provide an improved electromagnetic flowmeter. The above object is achieved by a flowmeter according to claim 9.

The underlying idea of the present invention is to overcome the aforementioned disadvantages of conventional electropolishing. The present invention incorporates a method of electropolishing which includes applying a high anodic DC voltage for a very short span of time to generate a very high current density at the surface of the electrode, particularly for small components (typically a few cm² in surface area) . This process may also be referred to as plasma polishing. The high current densities produced during plasma polishing lead to the formation of at least one reactive oxygen species (ROS) in the electrolytic solution. The ROS attacks the metal at the surface of the electrodes at the micro-peaks, but has a short lifetime (of about a few nano-seconds) , and hence does not attack the micro-valleys, which are less exposed to the solution than the micro-peaks. Unlike in conventional electropolishing, in the present method, no film is formed on the surface of the electrode. In one embodiment said first metal is chromium. It has been observed that the proposed electropolishing method is particularly advantageous for electrodes made of chromium containing alloys such as stainless steel.

In a preferred embodiment, the peak current density falls in the range of 500-1300 A/dm². It is still preferred that the time interval is less than 60 s . In a contemplated mode of operation, the DC voltage is adapted to produce an electrochemical potential of said electrode that is greater than 2000 mV with respect to standard hydrogen electrode. The above operating conditions, independently, or even advantageously, in combination, facilitate the formation of one or more ROS in the electrolytic solution to carry out the plasma polishing operation, while preventing excessive removal of material from the electrode head.

In one embodiment, the DC voltage is adapted to produce OH radical as an ROS. The OH radical attacks chromium metal from the surface directly because of its strong oxidation properties, thus converting chromium to dichromate in acid solution. The OH radical formed during plasma polishing also reacts to dissolve impurities like titanium carbide (TiC) and titanium nitride (TiN) from stainless steel.

In a further embodiment, said DC voltage is adapted to produce ozone (O3) as an additional ROS. Ozone advantageously reacts to remove impurities like titanium carbide (TiC) from stainless steel.

In another embodiment, said DC voltage is adapted to produce hydrogen peroxosulfate (HSO₅ ^") ion as an ROS. Hydrogen peroxosulfate (HSOs^~) ion, which is also able to thermodynamically to dissolve chromium metal into its dichromate in solution. The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1 is an electromagnetic flowmeter wherein the present invention can be incorporated,

FIG 2 is a schematic diagram of a set up for plasma polishing,

FIG 3 is an exemplary plot of current density versus time for plasma polishing,

FIG 4 is an exemplary Pourbaix diagram for chromium, and

FIG 5 is a graph illustrating the reduced flow measurement noise for an electromagnetic flowmeter having plasma polished electrodes .

The present invention provides an improved electropolishing technique, also referred to as plasma polishing. An exemplary embodiment of the present invention is described below wherein the proposed plasma polishing method is used for providing a polished surface for measuring electrodes of an electromagnetic flowmeter.

FIG 1 shows such a flowmeter 10 wherein the present invention can be incorporated. A fluid 14, whose flow rate is to be measured, flows through a measuring tube 12, along the direction of a longitudinal axis 26 of the measuring tube 12. The fluid 14 to be measured is electrically conductive, at least to a slight extent.

The flowmeter 10 includes a pair of measuring electrodes 16 and 18 arranged on opposite sides of the measuring tube 12 and in contact with the fluid 14. A magnetic arrangement is provided including electromagnets 20 and 22 that generate a magnetic field 24, oriented perpendicularly to the flow direction of the fluid 14. On account of this magnetic field 24, charge carriers in the fluid 14 migrate to the measuring electrodes 16 and 18 of opposite polarity. The potential difference which builds up across the electrodes 16 and 18 produces an electromotive force that is proportional to the flow velocity of the medium 14 averaged over the cross- sectional area of the measuring tube 12. A flow measurement means, such as a differential amplifier 32, amplifies this potential difference (i.e. the difference in the output signals 28 and 30 from the measuring electrodes 16 and 18 respectively) and provides the amplified output 34 to flow detection circuitry 36. The flow detection circuitry 36 calibrates the output 34 of the differential amplifier 32 to units of flow velocity or flow rate, and provides an output to output circuitry (not shown) .

The measuring electrodes 16 and 18 are metallic and hence prone to corrosion effect due to the fluid. To reduce the effect of corrosion, the surface of the electrodes need to be made smooth, since it is commonly known in corrosion science that a perfectly smooth and homogenous surface is more resistant to corrosion, than a non-homogeneous surface with many defects. The present invention proposes a technique of plasma polishing to smoother) the surfaces of the electrodes 16 and 18. As explained hereinafter, plasma polishing of the surfaces of the electrodes also increases the signal-to-noise ratio of the electromagnetic flowmeter.

FIG 2 schematically illustrates a set-up 40 for the proposed plasma polishing method. The electrode 16, having a surface 17 to be polished, is immersed in an acidic electrolytic solution 42. The acidic solution 42 may be, for example, a commercially available mixture of concentrated sulfuric acid and concentrated phosphoric acid. In the illustrated embodiment, the electrode 16 is made of stainless steel, or any other alloy containing chromium (Cr) . As illustrated schematically, the surface 17 is not smooth initially, and contains unevenness defined by micro-peaks 44 and micro- valleys 46. A positive DC voltage is applied from a source 48 to the electrode 16. For plasma polishing, the applied voltage is very high, producing a high current density at the surface 17 of the electrode 16 in a very short interval of time (particularly useful for small surface areas of the order of a few square centimeters) .

FIG 3 shows diagram 50 with a curve 58 of the current density ^λS' in A/dm² (represented along the axis 52) versus time ^λt' in seconds (represented along the axis 54) in an exemplary plasma polishing operation on stainless steel electrodes. In this embodiment, the peak current density 56 is around 1000 A/dm². In general, the preferable range of peak current densities for obtaining favorable results is 500-1300 A/dm². However, the invention can be carried out even at peak current densities in the range of 200-500 A/dm². The DC voltage is applied for a controlled interval of time ti. This is typically the time interval beyond which the curve 58 (current density v/s time) gets asymptotic with respect to the time axis 54. The time interval tl for application of the DC voltage is preferably small, and is 1 s in the illustrated embodiment. In general, a time interval of less than 10 s is preferable for the operation. Higher time intervals of exposure to the plasma polishing voltage (for example, time intervals exceeding 60 s) may lead to removal of material from the electrode head and thus alter the dimensions of the electrode head beyond acceptable tolerances. The same may be true for very high peak densities (i.e., much in excess of 1300 A/dm²) .

The inventive features of the present invention are explained with respect to an electrode made of stainless steel, or any other alloy containing chromium (Cr) . FIG 4 shows an exemplary Pourbaix diagram 60 for a Cr-S-H₂θ system at 25°C. The illustrated Pourbaix diagram 60 is a plot of electrode potential E of an electrode (in volts V), represented along the axis 62, and pH of the medium, represented along the axis 64. As known to one skilled in the art, a Pourbaix diagram maps out possible stable (equilibrium) phases of an electrochemical system. Predominant ion boundaries are represented by lines. In conventional electropolishing, gradual increase in the anodic voltage leads to the formation of Cr⁺², Cr⁺³ and finally a film made of complex chromium sulfate (Cr₂ (SO₄) ₃*14H₂0) . In the present invention, a very high potential will first of all cause formation of oxygen which changes the polishing mechanism totally. Unlike in conventional electropolishing, the formation of a film will not take place because a very intensive formation of oxygen will lower the pH (caused by formation of hydrogen ions) of the solution 42 according to the anode reaction represented by equation (1) below:

2H₂O = O₂ (g) + 4H⁺ + 4e^~ (1)

Working with a very high potential will also cause the formation of reactive oxygen species in the solution which are able to attack most materials. Reactive oxygen species (ROS) are ions or very small molecules that include oxygen ions, free radicals, and peroxides, both inorganic and organic. They are highly reactive due to the presence of unpaired valence shell electrons. In this embodiment, the ROS formed includes OH radical that is mostly formed on the tops of the micro-peaks 44. This is represented by equation (2) below.

H₂O = OH (radical) + H⁺ + e^" (2)

For the reaction in equation (2) to occur, a very high DC voltage is required, such that the electrochemical potential of the electrode 16 is greater than 2000 mV with respect to standard hydrogen electrode. In the illustrated embodiment the electrochemical potential of the electrode 16 is about 2700 mV with respect to standard hydrogen electrode. The OH radicals thus formed will only have a lifetime measured in nanoseconds, because of which micro-valleys 46 on the surface 17 will not be attacked by the OH radicals. On the other hand, the micro-peaks 44, being more exposed to the solution 42, will be attacked by the OH radicals. Because of the strong oxidation properties of the OH radical, at these micro-peaks, chromium (Cr) will be converted to dichromate (Cr₂O₇ ^"2) in the acid solution 42 as per the reaction represented by equation (3) below:

2Cr + 120H (radical) = Cr₂O₇ ^"2 + 2H⁺ + 5H₂O (Gibbs free energy, ΔG = - 666.616 kcal) (3)

Further advantageously, due to the formation of OH radical, nitrides will typically be converted to nitrates and metals in the highest oxidation state by reaction with OH radicals, as per the reaction represented by equation (4)

2CrN + 22OH (radical) ) + 2OH^" = Cr₂O₇ ^"2 + 2HNO₃ + 11H₂O (ΔG = -1036.996 kcal) (4)

Another advantage of the present invention is that, the polishing mechanism, being quite different from conventional electropolishing, is found to eliminate surface inclusions in the chromium containing base material. In particular, the present invention is found to remove inclusions like TiC, NbC, Nb₂C or Nb₃C, which are very difficult to remove with the conventional electropolishing process. Chromium containing alloys such as stainless steel normally contain such types of inclusions, because titanium (Ti) and niobium (Nb) are used for elimination of carbon in the steel. A lot of these inclusions are electrically conductive and hence contribute to so-called electrochemical noise, i.e., naturally occurring fluctuations in corrosion potential and corrosion current flow.

By conventional electropolishing it is not possible to remove, for instance, titanium carbide (TiC) or other inclusions on the surface of the electrode. The inclusions will probably anodic oxidized as per the reaction represented in equation (5) below. This reaction generally takes place for an electrochemical potential around 200 mV.

TiC + 4H₂O = TiO₂ + CO₂ (g) + 8H⁺ + 8e^" (5)

However, in the present invention, applying a significantly higher potential (using the plasma polishing) will make it possible to dissolve the TiC inclusions in a better way, because the higher potential makes if possible to form ROS like the OH radical which is able to dissolve the TiO₂ or oxidize TiC to a titanium peroxide ion (TiO (H₂O₂) ⁺²) as per the reaction represented in equation (6) below:

TiC + IOOH (radical) + 2H⁺ = TiO (H₂O₂) ⁺² + CO₂ + 5H₂O (ΔG = -582.887 kcal) (6)

Applying a high potential can additionally form further reactive oxygen species, for example ozone (O₃) , which can be further advantageous in removing TiC inclusions by oxidizing TiC to TiO (H₂O₂) ⁺² as per the reaction represented in equation

(7) below:

TiC + 1 2/3 0₃ (g) + 2H (+a ) = TiO (H₂O₂ ) ⁺² + CO₂ (ΔG = -304 . 244 kcal ) ( 7 )

Applying a high potential (plasma polishing) will furthermore cause the formation of more ROS like hydrogen peroxosulfate HSO₅ ^" ion, which also are able thermodynamically to oxidize Cr metal to dichromate according to the reaction represented by equation (8) below. Advantageously, hydrogen peroxosulfate is much more stable than the OH-radical.

12HSO₅ ^" + 2Cr + 4H⁺ = Cr₂O₇ ^"2 + 6H₂SO₈ ^" + 5H₂O (8)

The electrochemical potential for oxidizing hydrogen peroxosulfate (HSO₅ ^") to hydrogen peroxodisulfate (H₂SOs^") is calculated to be about 4000 mV (with respect to standard hydrogen electrode) at 50°C. The reaction for this oxidation is represented by equation (9) below:

2HSO₅ ^" + 3H⁺ + 2e^~ = HS₂O₈ ^" + 2H₂O (9)

It has been shown that it is possible to obtain electrode surfaces of unique quality after such a plasma polishing treatment as described above. FIG 5 shows a graph 70 obtained from flow measurements from plasma polished electrodes mounted on a test rig. The graph 70 plots the flow velocity ^λv' of the fluid as measured by the flowmeter in mm/s

(represented along the axis 72) versus time ^λt' (represented along the axis 74) . The standard deviations 76 of the measured mean flow velocity is noted on the graph. The standard deviation is a measure of flow measurement noise. In this case, for a mean flow velocity of 2600 mm/s a standard deviation of about 37 millimeters/second is observed. For non-polished electrodes, the usual values are about a factor 2 higher than what is observed for plasma polished electrodes.

The present invention thus provides electrodes for electromagnetic flowmeters that are more corrosion resistant than the type of electrodes currently used. Further advantageously, the present invention gives a better signal- to-noise ratio in the measurement circuit of the flowmeter due to the reduced corrosion activity at the electrode-fluid interface .

Summarizing, the present invention provides a method for polishing a surface of an electrode composed of at least a first metal. The method comprises immersing said surface of said electrode in an electrolytic solution and applying a positive DC voltage to said electrode. The positive DC voltage is applied for a controlled time interval to generate a peak current density at said surface that causes formation of at least one reactive oxygen species (ROS) in said solution. The ROS is reactive to dissolve said first metal from micro-peaks at said surface of said electrode to smoothen said surface. The present invention further provides an electromagnetic flowmeter having measuring electrodes having surfaces that are polished by the above method.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined by the below-mentioned patent claims.

Claims

Patent claims

1. A method for polishing a surface (17) of an electrode composed (16) of at least a first metal, said method comprising:

- immersing said surface (17) of said electrode (16) in an electrolytic solution (42), and

- applying a positive DC voltage to said electrode (16) for a controlled time interval (ti) to generate a peak current density (56) at said surface (17) that causes formation of at least one reactive oxygen species (ROS) in said solution (42), said ROS being reactive to dissolve said first metal from micro- peaks (44) at said surface (17) of said electrode (16) to smoothen said surface (17) .

2. The method according to claim 1, wherein said peak current density (56) falls in the range of 500-1300 A/dm².

3. The method according to claim 1 or 2, wherein said time interval (ti) is less than 60 s.

4. The method according to any of the preceding claims, wherein said DC voltage is adapted to produce an electrochemical potential of said electrode (16) that is greater than 2000 mV with respect to standard hydrogen electrode.

5. " The method according to any of the preceding claims, wherein said first metal is chromium.

6. The method according to claim 5, wherein said DC voltage is adapted to produce OH radical as an ROS.

7. The method according to claim 5 or 6, wherein said DC voltage is adapted to produce ozone (O₃) as an ROS.

8. The method according to any of claims 5 to 7, wherein said DC voltage is adapted to produce hydrogen peroxosulfate (HSOs^") ion as an ROS.

9. An electromagnetic flowmeter (10), comprising:

- a measuring tube (12) through which a fluid (14) to be measured flows, means for generating a magnetic field (24) perpendicular to a direction of flow of said fluid (14), and a pair of electrodes (16,18) which pick up an electromotive force generated according to a flow rate of the fluid (14) , wherein each of said electrodes (16,18) is composed of at least a first metal, and characterized in that each of said electrodes (16,18) has a surface that is polished by a method according to any of claims 1 to 8.