WO1992016673A1

WO1992016673A1 - Method and arrangement to hinder local corrosion and galvanic corrosion in connection with stainless steels and other passive materials

Info

Publication number: WO1992016673A1
Application number: PCT/NO1992/000046
Authority: WO
Inventors: John M. Drugli
Original assignee: Sinvent A/S
Priority date: 1991-03-13
Filing date: 1992-03-12
Publication date: 1992-10-01
Also published as: NO910993L; NO910993D0; NO312731B1

Abstract

The invention concerns device and method to prevent local corrosion on different types of stainless steels and other passive materials, caused by the strongest oxidizer in a solution containing oxidizers, especially chlorinated seawater, whereby the passive materials are polarized by a constant applied cathodic current with a cathodic current density which is at least as high as the maximum diffusion controlled cathodic reaction current caused by the strongest oxidizer in the solution, so that the potential increase this oxidizer otherwise would lead to, is counteracted. As anode can be used a sacrificial anode which is in contact with the solution and insulated from the construction to be protected, except for connection through a previously calculated electrical resistance which restricts the current; or a DC power supply and a permanent micro anode can be used. The invention leads to less consumption of sacrificial anode compared to the consumption which is usual if the current is not restricted.

Description

Method and arrangement to hinder local corrosion and galvanic corrosion in connection with stainless steels and other passive materials.

The invention includes a method and an arrangement to hinder/restrict local corrosion on high alloyed stainless steels in chlorinated sea water and other water containing fluids where the strongest oxidizer, which always is the main reason for the initiation of local corrosion, is present in small concentrations. The cathodic reaction rate for this oxidizer is presupposed considerably lower than the reaction rate for the other cathodic reactions taking place on stainless steels in the medium when conventional methods for cathodic protection are used. The invention can also hinder galvanic corrosion when certain materials are coupled to passive materials as stainless steels, titanium and nickel alloys. The invention eliminates the harmful effect of the strongest oxidizer with respect to initiation of local corrosion and gives protection by very low cathodic current density and by very low anode consumption. The invention include sacrificial anodes with built-in resistors to restrict the current output and to increase the lifetime of the sacrificial anodes.

The invention expands the possible application field of different types of stainless steels and some other passive materials with respect to temperature and concentration of aggressive species. For example can actual types of materials, which to-day are used in chlorinated sea water, be used at higher temperatures and with higher concen¬ trations of chlorine in the water than before. In systems for transportation and treatment of produced water in the oil and gas industry the invention can hinder attack on conventional stainless steels at high temperatures even with small amounts of oxygen in the system.

Stainless steels in environments with aggressive ions (i. e. chloride ions) suffers of local corrosion if the surface electrode potential is raised above the critical potential at a given temperature, or if the temperature is raised above the critical temperature at a given potential. The critical temperature and critical potential varies dependent of the type of alloys, concentration of aggressive ions, of design a.o. Certain oxidizers in the water solution can increase the surface potential above the critical potential and in that way initiate local corrosion.

In electrolyte systems with small concentrations of the strongest oxidizing agent the invention can hinder the potential increase (that leads to corrosion initiation) by use of small cathodic currents.

One example on a system where the strongest oxidizer is present in small concentrations is chlorinated sea water. Chlorine is added to prevent biological fouling. Sea water usually also contains oxygen as an other oxidizer. Chlorine, however is a much stronger oxidizer and can rise the electrode potential considerably above the potential in unchlorinated sea water (see Figure la) . The cathodic current density caused by the chlorine reaction in the upper part of the potential region is very small. The invention^ purpose is to counteract the potential increase caused by the chlorine reaction and to keep the potential at the same level or lower than in unchlorinated sea water (without biological slime layer) . The required cathodic polarization current density is usually very small and might be achieved by use of sacrificial anodes with a built- in resistor to restrict the current output or by use of DC power supply and a small permanent anode.

The current required to give the intended effect is low, this means that the consumption of the sacrificial anodes is low if the type of anodes described above are used, and that the ohmic drop in the electrolyte also would be low. This makes it possible to protect large surface areas from each anode. In systems containing produced water where dissolved C0₂ makes the water aggressive with respect to carbon steel, but not to stainless steels, small amounts of oxygen make the water aggressive also for stainless steels. The oxygen is here the strongest oxidizer in the system and involves a risk for local corrosion attack on stainless steels if the temperature is too high. The effect of the small amount of oxygen can be counteracted by a very small cathodic current and a very small anode consumption compared to cathodic protection by use of conventional cathodic protection methods.

The invention can in the same way as described above also counteract the strongest oxidizer in other systems by use of small cathodic currents.

Cathodic protection of stainless steels is a well known technique, however it is not very much used. One reason is that if protection by use of sacrificial anodes is applied, the anode consumption by use of ordinary anodes of zinc or aluminum in many systems often is of the same order as for protection of carbon steel.

Cathodic protection by use of applied DC current controlled by the electrode potential measured by a reference electrode is also a possible protection method. This method, however, is sensitive because a failure in the relatively complicated electronic equipment, DC power supply or the reference electrode may give very bad results.

Because of relatively high costs by mounting, maintenance and service, cathodic protection of stainless steels is not much used. The problems have instead usually been solved by selection of still more corrosion resistant, but also more expensive materials.

The object of the invention is to reduce the risk for local corrosion and galvanic corrosion for certain materials, for example copper alloys, coupled to stainless steels, titanium and nickel alloys and to expand the field of application for different types of stainless steels and other passive materials with respect to temperature and concentration of aggressive agents in solutions where the strongest oxidizer, which is the main cause of local corrosion, is present in small concentrations. The aim is to obtain this in a simpler and cheaper way than by the methods previously used.

The object according to the invention can be achieved by use of an approximate constant cathodic current applied on the surfaces. The current has to be at least as high as the diffusion controlled cathodic reaction current for the oxidizer giving the most noble potential in the solution and to counteract the potential increase caused by this agent. Because of the low reaction rate of this agent in many systems the required cathodic current density to eliminate the effect also is low. Low current density in solutions with low resistivity means low potential drop between anode and cathodic surfaces, which makes it possible to protect large surfaces with large distance between the anodes.

For in a safe and easy way to polarize the surfaces with the required cathodic current density, but without getting un¬ necessarily high current, a sacrificial anode insulated from the object that is to be protected is used, the only electrical connection is over a resistor that restrict the current to the required level. The anode shown in Figure 2 is designed to be mounted externally on pipes or vessels and is designed to stand up with very rough handling. The anode in Figure 2 is suited for protection of chlorinated sea water systems.

The required cathodic current density i_c should be determined by long duration potentiostatic polarization tests of the actual material in the actual environment. If the surface area A_c that are going to be protected is known, the required current from the sacrificial anode can be calculated:

A. Required weight of the anode in kg can be calculated by use of the common formula for calculation of anodes:

_{Q =} J(--i) - 8760 (h/year) ^• n (year) B (Ah/ kg) ^• η

B = Efficiency of the anode material in Ah/kg η = Utilization factor n = Service time in years

Total electric resistance in the circuit is:

R = R-_. + R_a

R_a = The resistance in the electrolyte R-, = The electric resistance built into the anode holder

-'^■a

E__ = The electrode potential of the passive material polarized to the required level (see Figure 1)

E_a = The electrode potential of the sacrificial anode

R_a depends on the design of the anode. For the type of anode shown in Figure 2 R_a mainly are determined of the diameter of the hole in the wall of the pipe or vessel that is protec¬ ted. R_a can be determined by calculations or by testing.

Because of the form of the cathodic polarization curves in such systems E__. is approximately constant. E_a is nearly independent of the current output. The result is that I_a is appointed when R is determined. However, I_a should always be higher than the limiting current for the cathodic reaction for the strongest oxidizer in the solution (see Figure 1) . If zinc is selected as the material in the sacrificial anodes E- - E,. = about 1.0 Volt in chlorinated sea water. For R = 100 Ohm the current output from the anode will be close to 10 mA. This is sufficient to reduce the potential on high alloyed stainless steels 500 to 600 mV on a surface area up to 10 to 50 m² dependent on the level of chlorination, the flow rate in the solution and the geometrical factors. The anode consumption will be very low, this means that small anodes can be designed for long lifetimes. To a certain degree the system with the sacrificial anodes is self-regu¬ lating because the current is proportional to E_x - E_a. In accordance with the invention the risk for initiation of local corrosion on passive materials can be reduced consi¬ derably in a safe and cheap way in electrolyte systems where the strongest oxidizer is present in small concentrations. The invention makes it possible to reduce the required material quality with respect to corrosion resistance for materials applied and to reduce the risk for galvanic corrosion for certain materials as for example on Cu-alloys coupled to stainless steels, titanium and nickel alloys.

In the following the invention will be described based on examples of design and with references to enclosed figures, where Figure 1 shows cathodic polarization curves for passive stainless steels, in a) chlorinated see water and b) produced water with small amounts of oxygen. In fig. 1 the abcissas show the potential and the ordinates show the log of teh cathodic current; the symbols have the following meaning:

E_a = The anode potential;

E_x = Potential on stainless steel close to the anode;

E_orit = The critical potential for initiation of local corrosion; E_max = Max potential on a passive stainless steel surface; I_a = Anode current;

E__rit - E- = Max. driving voltage in the electrolyte. R = R_a + R-_ = Anode resistance in the electrolyte plus the electrical resistance between anode and protected object. Cl₂-red. Chlorine reduction 0₂-red. 0₂ reduction

H⁺-red. H* reduction

Figure 2 shows a cross section through a sacrificial anode with restricted current output mounted externally on a pipe or a vessel, and the numbers have the following meaning: 1) Pipe/vessel wall 2) Connection tube 3) Anode 4) Insulating lid 5) Nut 6) Washer

7) Washer 8) Gasket

9) Gasket 10) Ring of metallic material

11) Bolt 12) Insulating tube

13) Steel bolt 14) El. resistor 15) Lid of insulating material

Figure 3 shows a cross section through a permanent anode (micro anode) , and the numbers have the following meaning:

1) Pipe/vessel wall 2) Insulating insertion 3) Permanent anode 4) Connecting tube

5) Plastic resin 6) Screw

7) Bolt 8) DC power supply 9) Connecting leads

Figure 4 shows a cross section through a sacrificial anode mounted internally in a vessel with the resistor that restricts the current output located externally. The numbers have the following meaning:

1 Vessel wall 2) Connecting tube with flange

3 Insulating insertion 4) Sacrificial anode

5 Flexible cable welded or brazed to anode and bolt

6 Bracket 7) Bolt

8 Insulating bushing, plate and washers

9 Bolt 10) Washer

11 Nut 12) El. resistor

13 Ring of metal 14) Gasket

15 Gasket 16) Lid

17 Bolt Description of the protection system.

The figures la and lb show examples on how the protection system is working. In chlorinated sea water it is the reduction of chlorine that rises the potential on stainless steels to the maximum level E_max: shown in Figure la. For certain conditions E_ma-. might be more positive than the potential that leads to initiation of local corrosion on stainless steels. The maximum allowed potential for a given material is indicated with the potential E_crit. By applying a small constant cathodic current indicated by I_a, the elec¬ trode potential of the material will be reduced from E_ma3- to E_x which should be more negative than E_crit. I_a has to be higher than the actual cathodic limiting current that should be counteracted. I_a is under almost all service conditions several decades lower than the cathodic limiting current for the oxygen reaction I₀₂ indicated in Figure la.

Because of ohmic voltage drop in the electrolyte the poten¬ tial will be more positive with increasing distance from the anode. The anodes have to be located in a way that secures that the electrode potential of the protected material never exceeds E_crit at any place. The largest allowable distance between the anodes is dependent on geometrical factors, flow rate and resistivity of the electrolyte. The distance has to be calculated based on the given conditions in each case.

Examples of anode design.

Figure 2 shows how a sacrificial anode for protection of stainless steels in chlorinated sea water or chlorinated brines can be made. The anode is mounted externally on a vessel or a pipe of stainless steel, a hole with the dia¬ meter d is drilled in the wall of the vessel/pipe. A connecting tube with a flange 2 of the same material as the vessel/pipe is welded to the wall. The connecting tube have to be mounted in a way that allows air and gas to escape. The diameter d can be varied dependent of the anode resis¬ tance wanted. For special purposes the diameter of the hole can be expanded to D and thereby the anode might be extended into the vessel/pipe. The anode 3 might for chlorinated water be made of zinc. In other environments other anode materials might be used. The anode is fixed to a steel bolt 13 which again is fixed to a lid of insulating material 4 with a nut 5 and a washer 6. In the insulating lid a steel washer 7 and a gasket 8 are mounted for tightening, between the insulating lid 4 and flange 2 there also is a gasket 9. The insulating lid is pressed against the flange with the clamping ring 10 of a electric leading material by use of the bolts 11. An insulating tube 12 is inserted between the connecting tube and the anode material to prevent direct me¬ tallic contact between anode and structure. On top of the anode bolt 13 it is drilled a small hole in which the elec¬ trical resistance 14 is connected by soldering or in another safe way. The other end of the resistance is connected to the clamping ring to secure the electrical connection. Above the space with the electrical resistance there is a lid of insulating material to protect the resistance. To record the current flowing from the anode, and to record the potential on the protected material at service conditions the lid can be removed. The potential is recorded by measuring the voltage between the anode bolt 13 and the structure 1. By taking the anode resistance in the electrolyte into account the electrode potential on the protected surfaces (with zinc as reference) can be calculated for the surface areas close to the anode. For small values of R_a compared to the total resistance R, the potential with sufficient accuracy can be read directly without any correction. The current is recor¬ ded by measuring of the potential drop over the resistance 14 (R .

In that the anodes are easy exchangeable, in some cases it can be suitable to use still smaller anodes than indicated in Figure 2. If the anodes are exchanged every second year, an anode weight of a few hundred grams usually can be used in chlorinated sea water.

In Figure 3 there is a sketch of a system with a permanent anode and a DC power supply. The anode might be made of suitable surface treated titanium, platinated titanium or other suitable materials. The anode 3 is mounted somewhat recessed from the surface to protect the anode against mechanical damages. The anode is insulated from the main structure that shall be internally protected by use of an insulating insertion 2 fixed to a flange 4 by bolts 7. A lead is connected to the anode with a screw 6. The connec¬ tion is protected by filling a plastic resin 5 into the space around the screw. The lead is connected to the plus pole on the power supply which shall deliver the required constant amount of DC current I_a. The minus pole of the power supply is connected to the object that shall be protected.

The use of permanent anodes and DC power supply for cathodic protection is a well known technique. The difference between this method and the other is that this method by use of micro anodes and small currents is able to protect large areas of passive materials in certain environments.

For certain purposes it can be necessary to mount the anodes internally for example in a vessel. This can be done as indicated in Figure 2, by use of full opening D and an extended anode. However the anodes might also be fixed to brackets internally with insulation between the anodes and the brackets. An insulated rod of for example steel is guided through the wall and over an electrical resistance R-_ externally connected to the vessel as shown in Figure 4. The anode current can be recorded by measuring of the voltage over the known electrical resistance in the same way as explained for the design in Figure 2. The surface electrode potential internally in the vessel can also be measured with zinc as reference.

The sacrificial anode as reference electrode To measure the electrode potential of a metal surface in chloride solutions zinc might be used as reference material, because the electrode potential of pure zinc and anode material of zinc is relatively stable. Especially it is so when a small anodic current is applied on the material. By use of the common type of cathodic protection the anode usually are connected directly to the object that shall be protected. The anode current is then large and the result is a large voltage drop between the anode and the protected surfaces in some distance from the anode. It is not possible to calculate this voltage drop because the anode current is not known. To measure the electrode potential of the pro¬ tected surfaces special reference electrodes of for example zinc, silver/silver chloride or copper/copper sulphate have to be used.

When the current from the anode is restricted by use of an electrical resistor as indicated in Figure 2, the anode current very often will be so small that the voltage drop in the electrolyte between anode and the protected surfaces can be neglected. The potential can then be read directly by use of volt meter connected to the anode and the structure. In some cases, however, the voltage drop will have some in¬ fluence. The recorded potential can then be corrected be- cause the anode resistance in the electrolyte R_a is known for a given design (if not, R_a can be calculated or found by testing) . The anode current can be recorded by measuring the voltage drop dE over the known resistor R_,.

I. = dE/R-,

The potential on the protected surface with the anode material as reference is then:

E = The potential read (=I_aR-_) + I_aR_a

I_a is proportional to the anode consumption and can be used to calculate the consumption by use of Faraday's law.

Claims

PATENT CLAIMS

1. Method to prevent local corrosion on different types of stainless steels and other passive materials, caused by the strongest oxidizer in a solution containing oxidizers, espe¬ cially chlorinated sea water which contains small amounts of chlorine, and produced water in connection with oil and gas production which may contain small amounts of oxygen, where the passive materials are polarized by a constant applied cathodic current, c h a r a c t e r i z e d i n that the applied cathodic current is at least as high, however not significantly higher than the maximum diffusion controlled cathodic reaction current caused by the strongest oxidizer in the solution, so that the potential increase this oxidizer otherwise would lead to, is counteracted, whereby it is used to create the applied cathodic current

1) a sacrificial anode which is in contact with the solution and which is connected to the materials that shall be protected with an electron leading connection, whereby, for restriction of the current, so that unwanted anode consumption can be prevented, in the connection between the sacrificial anode and the material a previously calculated electrical resistance is put which restrict the applied cathodic current, so that the current does not be essen¬ tially higher than the current required to prevent the rise of potential caused by the oxidizer, or

2) a DC power supply and a permanent micro anode.

2. Arrangement to prevent local corrosion on stainless steels and other passive materials, caused by the strongest oxidizer in an oxidizing solution, especially sea water which contains small amounts of chlorine, and produced water in connection with production of oil and gas with small amounts of oxygen, whereby the arrangement includes a sacrificial anode which is in contact with the solution and which can be connected to the materials that shall be protected with an electron leading connection in a way that makes it possible to polarize the passive materials with an applied cathodic current at least as high as the maximum diffusion controlled cathodic current caused by the strongest oxidizer in the solution, so that the potential rise otherwise caused by this oxidizer is counteracted, c h a r a c t e r i z e d i n that to prevent unwanted anode consumption, in the current circuit it is put a previously calculated electrical resistance which restrict the cathodic current, so that the current not will be essentialy higher than the current required to counteract the rise of potential caused by the strongest oxidizer.

3. Use of the arrangement according to Claim 2 for recording of surface potential on a metallic structure by direct measurement of the electrical voltage between the sacrificial anode and the structure.