C2012-0001401
Cathodic Protection of Stainless Steels and other Corrosion Resistant Alloys
Jan Heselmans
Corrodium bv
Planetenweg 5
NL-2132 HN Hoofddorp
René Wouts
Corrodium bv
Planetenweg 5
NL-2132 HN Hoofddorp
ABSTRACT
By now it is common knowledge that stainless steels ranging from UNS S31600 up to 6%
Molybdenum stainless steels and (super)duplex stainless steels do not always resist seawater.
Even for fresh natural water it is well known that 2% Molybdenum stainless steels such as UNS
S31600 may fail if aggressive biofilms can develop. Seawater resistant copper alloys, such as UNS
C70600 and UNS C63000 may fail due to high flow effects. Improper chlorination can cause damage to all above-mentioned alloys. And last but not least, these corrosion resistant alloys
(CRA's) can cause galvanic corrosion to nearby steel constructions, or suffer galvanic corrosion if contacted to nearby noble behaving metals such as Titanium. This paper describes cathodic protection options for protecting stainless steels and other CRA’s against localized corrosion and for elimination of galvanic corrosion. The subject 'Anodic Protection' will be discussed as well.
Key words: Stainless steel, titanium, corrosion resistant alloys, seawater, cathodic protection, galvanic corrosion, pump caisson, sacrificial anode, pulsing anode, potential control unit, anodic protection, sulfuric acid,.
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

INTRODUCTION
Stainless steels and other CRA´s are widely used in water systems such as seawater, brackish water, river water and well water. Seawater and brackish water are corrosive because of the high chloride (salt) content and due to formation of a corrosive biofilm with untreated natural water. The fresh natural waters can be corrosive due to formation of a corrosive biofilm. Copper alloys such as Cunifer (UNS C70600) and NiAlbronze (UNS C63000) resist seawater and all other waters very well in stagnant or low flow condition. But at high flow rates such alloys are sensitive to erosion corrosion.
Galvanic couplings of stainless steels to more noble alloys such as Titanium can increase corrosion problems strongly, especially in waters with high specific conductivity such as seawater. On the other hand, stainless steels can cause severe galvanic corrosion to carbon steel constructions, where the best example is very high corrosion rates of steel caissons caused by duplex stainless steel or other CRA’s in seawater liftpumps and risers. Floating production, storage and offloading platforms (FPSO) suffered leaks in the caisson (hull) within 2 years time which resulted in repair costs over USD 2,000,000.
Cathodic protection using actively controlled sacrificial anodes can completely stop the corrosion problems, thus assuring optimal lifetime for pumps, caissons, valves, tubular heat exchangers and piping systems.
CORROSION AND PROTECTION POTENTIALS
In contacted with water, the alloy’s potential will increase due to extraction of electrons from the alloy in accordance with the cathodic corrosion reaction:
O
2
+ 2H
2
O + 4e → OH
–
For example, in seawater, UNS S31603 stainless steel will change from -100 mV Ag/AgCl to the pitting potential of approximately +250 mV Ag/AgCl. Natural aerobic biofilms will accelerate reaction
[1]. In this way natural waters will cause pitting corrosion more easily than sterile water (chlorinated water). If chlorinated, the concentration of free chlorine should not be higher than 1 ppm for duplex stainless steel as hypochlorite by itself also is corrosive to stainless steels and copper alloys.
Cathodic protection keeps the alloy in a safe (more negative) potential region by supplying electrons to the alloy susceptible to corrosion. For Zinc anodes the reaction is as follows:
Zn → Zn
2+
+ 2e
Cathodic protection is not only achieved with Zinc but also with aluminium, magnesium or impressed current cathodic protection (ICCP) Table 1 below gives the free potentials and protection potentials of several alloys and galvanic couples. A free potential is the potential of an unprotected alloy measured with a reference electrode such as a silver-silver chloride reference electrode
(Ag/AgCl reference probe).
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

Table 1
Rest Potentials and Protection Potentials of Several Common Seawater Alloys
Alloy/Galvanic
Couple in
Seawater
Stainless steel
UNS S31603
Duplex stainless steels
Super duplex stainless steels
6% Molybdenum stainless steels
Steel
Copper and bronze alloys such as
UNS C70600
UNS C63000
Free potential in chlorinated seawater mV Ag/AgCl
-100 up to
+250 (pitting zone)
-100 up to
+250
-100 up to -
300
Free potential in natural sea water mV Ag/AgCl
0 up to +250
(pitting zone)
-100 up to +400
(pitting zone)
-100 up to -300
Protection potential mV Ag/AgCl
-400 down to
-700 depending on biofilm activity and temperature
-300 down to
-600
0 up to +250 -100 up to +500
(pitting zone)
0 up to +350
-450 down to
-600
0 up to +500
(pitting zone)
-400 down to
-600
-200 down to
-600 depending on biofilm activity and temperature
-200 down to
-600 depending on biofilm activity and temperature
<-800 or <-900 in case of MIC
<-700
Over protection at mV Ag/AgCl
<-900 for cold worked zones under high stress.
<-900 for welding zones with high stresses
<-900 for welding zones with high stresses
<-1200 for zones with high cold deformation.
<-900 for high strength steel welding zones
-
Similar to carbon steel, stainless steels and copper alloys can be 100% cathodically protected. The cathodic protection design must be properly done using international standards such as DNV RP
B401 and computer models for calculating anode resistance, anode currents, protection potentials and anode spacing.
For optimum cathodic protection, enough space and electrolyte column is required. Anodes must be distributed properly over the surface to be protected in order to do their protective function. The anode spacing depends on the pipe or vessel dimensions or annulus width. For example hanging anodes are used in the annulus pump/caisson. (see Figure 7).
ELECTRONIC CONTROL OF SACRIFICIAL ANODES
Corrodium
1
installed anodes are controlled using an electronic circuit board. In order to eliminate any threshold voltage or internal resistance, field effect transistors (FET'S) are used. The FETs are powered by a capacitor, which is charged by the sacrificial anodes and the cathode (the metal to be protected). If the resistance of the anode/cathode is too high, for example due to coating of the cathode, a 9Volt battery is used for controlling the sacrificial anodes. The power consumption of the circuit board is very low which allows a lifetime of 5 years for the 9 Volt battery. Electronic control for
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the
1
Corrodium is a Tradename author(s) and are not necessarily endorsed by the Association.

1. Limiting the current by pulsing results in a reduction of anode consumption. Microbiological influenced corrosion (MIC) is reduced because of alternating pH-value on the surface film. For this application a circuit board is used to create a pulsed voltage with a pulse duration of 5-60 minutes, and a pulse amplitude from -100 and -800 mV AgAgCl (see Figure 1).
2. In some cases over-protection can lead to damage. Such as Hydrogen damage to duplex stainless steels, or Hydrogen blistering of coatings.
800
700
600
500
400
300
200
100
0
1 2 3 4 5 time
Figure 1: Automatic potential pulse caused by magnesium sacrificial anode on stainless steel sandbed filter.
For elimination of galvanic corrosion the protection potential is controlled at a set value against a
Ag/AgCl reference electrode or a zinc reference probe. During alternating conditions such as variations in flow, temperature or scaling formation the potential is kept at a constant ideal value.
This assures the ideal protection potential and avoids over protection which can result in hydrogen embrittlement or hydrogen blistering of coatings . (see Table 2).
Excessive hydrogen formation or high anode consumption rates also can be avoided by using serial switched resistors or diodes. Diodes cut the anode current because of their threshold voltage.
Resistors cut the anode current by increasing the total system resistance. However, as resistors and diodes are static components, they do not react on alternating conditions such as flow (Figure 2) or temperature.
Table 2
Protection Potential for Galvanic Couples
Galvanic couple Alloy to be protected Set protection potential mV Ag/AgCl
Steel -850 Steel /
Stainless Steel
Coated steel /
Titanium
Steel -850
Stainless Steel
/ Titanium
Copper alloy UNS
Stainless Steel
UNS C63000
-500
-600
Set protection potential mV Zinc/seawater
+150
+150
+500
+400
Stainless steel
/ UNS C63000
-600 +400

950
850
750
650 automatic conventional
550
0 2 4 6
Flow Rate (m/s)
8 10
Figure 2: Automatic Potential Control versus conventional sacrificial anodes with a resistor.
The static resistor does not react on alternating flow rates whereas the PCU unit (potential control unit) does.
APPLICATIONS
Protection of (Super) Duplex Stainless Steel and Copper Alloy Seawater Coolers
Marine vessels and for coastal industrial cooling applications, often are cooled with seawater. For tubular heat exchangers a common materials selection is duplex or super duplex stainless steel.
Normally the seawater is on the tube side as this side is less sensitive to corrosion than the shell side. However, a common source of damage is the tube sheet/waterbox crevice causing crevice corrosion (see Figure 3). Installing pulsing anodes in the water box will stop the crevice corrosion or any other corrosion. Normally the protection range of the tubes is limited to 5x the diameter of the tubes. However, hardly any corrosion on the tubes is seen, the crevices are the real problem.
With many crevices between tube sheet and tubes, the shellside is even more sensitive to crevice corrosion than the tubeside. For that reason, normally the tubeside is selected as 'seawater side'. If for whatever reason the shell side has been selected as seawater side, installation of electronic controlled rod-anodes is recommended. Also coolers made of copper alloys are protected against erosion corrosion at the inlet and outlet by using controlled sacrificial anodes.
Figure 3. Left: Crevice corrosion near the pipe welds of a seawater/ammonia cooler (duplex stainless steel 22%Cr/5%Ni). Right: Flanged anode with control box for pulsing aluminium anode.
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

Protection Of Duplex SS Piping Against Galvanic Corrosion
Galvanic corrosion can occur near the titanium/duplex SS connection of a gas cooler in seawater.
High pressure flanged anodes were installed near the cooler in order to protect the piping and nearby valves (see Figure 4).
Figure 4: High pressure flanged zinc anodes with enclosure (item 3) for electronic control of anode current.
Protection Of Coated Steel And Valves In Contact With Titanium Heat Exchangers.
On the connection of a titanium sheet heat exchanger the titanium sheets caused galvanic corrosion on microscopic small coating defects in coated steel pipes. In addition, nearby copper alloy UNS
C63000 valves were leaking within 2 months after start up of the installation. Electrical insulation using Nylon spacers and rings, plus installation of zinc sacrificial anode rods provided 100% protection (see Figure 5).
Figure 5: Severe galvanic corrosion of a copper cast alloy (UNS C63000) valve and coated carbon steel piping caused by the titanium seawater coolers. The coolers were insulated from the piping and zinc rod anodes were installed in the 1 inch instrument welding necks.
The grey pipes are seawater pipes the white pipes are cooling water pipes for cooling the ship engines.
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

Protection Of Seawater Pumps/Risers And The Carbon Steel Caisson Surrounding.
Seawater lift pumps and other seawater pumps are very delicate and expensive equipment. Often corrosion of the duplex or super duplex stainless steel motor housings or pump bowls is seen.
Corrosion can be caused by over dosing of sodium hypochlorite (chlorine) (see Figure 6).
Another major problem is severe galvanic corrosion of the steel caisson. Corrosion rates of >10 mm/year were reported, causing leakages in FPSO (floating oil platforms) hulls and millions of USD repair costs. Many pumps and caissons nowadays are protected by sacrificial anodes with a potential control unit (PCU). Measurements and finite element calculations in pump caissons demonstrated that the anodes must be well distributed in the annulus pump/caisson [1]. For this reason the (patented) hanging annulus anodes were developed. Figure 7 shows installation of potential controlled hanging anodes on a 800 KW electrical pump. The anodes fit in an annulus of only 62 mm. By using Nylon bolts, rings and nuts as 'spacers' the anode position in the annulus can be exactly adjusted.
Figure 6: Left: Pitting corrosion and leakage on seawater lift pump motor enclosure (super duplex SS) causing short circuit to the 800 KW motor. Right: Pitting corrosion in motor end
(super duplex SS cast). The corroded area was near the hypochlorite injection system, causing over dosage of hypochlorite on these locations.
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

Figure 7: Seawater liftpump with anodes installed from top to down at the motor end. Upper right cylindrical enclosure: subsea potential control unit. Picture right: Pump with anodes descending in caisson. Note the limited annulus space of only 62 mm. The controlled sacrificial anodes protect both the steel caisson as duplex stainless steel pump and riser.
Protection Of UNS S31603 Sandbed filters. Magnesium Pulsing Anodes In Fresh Water.
At ideal MIC temperature of around 30 °C UNS S31603 sandbed filters are very sensitive to biofilm formation and MIC. See figure 8. Installation of one pulsing Tank Magnesium Anode completely stopped all corrosion. It is now a standard practice to install pulsing Mg Anodes on stainless steel sandbed filters.
Figure 8: Left: MIC (microbiologically influenced corrosion) in sandbed filter made of stainless steel UNS S31603. Right: Pulsing magnesium tank anode in sandbed filter. A printboard capacitor is charged by the anode and cathode (stainless steel) 'water battery'.
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

Anodic Protection
Just as cathodic protection of stainless steels is used in water environments, anodic protection is used in sulfuric acid processes. In sulfuric acid, stainless steel corrodes uniformly in the active region. This in contrary to seawater environments where the stainless steel corrodes locally (pitting) at potentials higher than the passive region, the pitting potential.
For anodic protection the stainless steel anode (+) is protected by a cathode (-) using impressed current power supplies. Instead of forcing the system in the negative direction, the stainless steel is artificially repassivated by forcing it in the noble direction; several hundreds of millivolts above the value of a Platinum reference probe. Figure 9 shows a sulfuric acid heat exchanger protected by an anodic protection impressed current unit.
Figure 9: Sulfuric acid heat exchanger (stainless steel UNS S31603 protected by anodic protection system. Middle of heat exchanger: Connection to cathode bar (- pole), made of nickel base alloy (UNS N10276). The (+ pole) is connected to the stainless steel heat exchanger and pipe bundle.
INSPECTION AND MONITORING
This delicate and expensive equipment can be inspected or monitored by using Ag/AgCl half cells or
Zinc reference probes. For anodic protection Platinum reference probes are used. Often the probe is permanently installed on a strategic location. For the seawater liftpumps this is near the pump inlet. Often once a month a potential measurement is done but in this way, online monitoring to the
DCS is possible as well. An alarm can be set if the potential steps out of the specified bandwidth.
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

CONCLUSIONS
• Cathodic protection (CP) of stainless steels and other CRA's in seawater, brackish water, well water, and other (natural) fresh waters can be very effective and useful. CP mainly is used for equipment with larger dimensions and diameters.
• For best cathodic protection, sufficient space and electrolyte (water) column are required. Anodes must be distributed properly in order to provide adequate protection. The anode spacing depends on the pipe diameter or annulus width.
• For process systems such as pumps, heat exchangers, valves and piping systems electronically controlled sacrificial anodes are more effective than impressed current power supplies.
• Pulsing anodes fully protect the stainless steel and also save anode consumption and help to reduce biofilm formation and MIC.
• For elimination of galvanic corrosion, potential controlled sacrificial anodes are applied. They guarantee the best protection potential and avoid over-protection and
Hydrogen problems on duplex stainless steel, high strength steels and coatings.
• Anodic protection works very well for protecting stainless steel in sulfuric acid processes. For anodic protection, impressed current power supplies (ICCP) are used however, compared to cathodic protection units the positive and negative poles are inverted.
REFERENCES
3
4
1
2
Jan Heselmans, Ko Buijs, Ephraim Isaac: Sacrificial anodes for protection of seawater pump caissons against galvanic corrosion. NACE Corrosion 2011 paper 11056.
Svenn Magne Wigen, Harald Osvoll: Corrosion problems in seawater pump caissons.
Practical solutions.
P. Woolin, W. Murphy: Hydrogen embrittlement stress corrosion cracking of super duplex stainless steel. Corrosion 2001 (NACE).
R. Francis, G. Byrne, and G.R. Warburton: Effects of cathodic protection on duplex stainless steel in seawater. ISBN 97030234 CJ
©2012 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International,
Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.