The article is about the Design of Cathodic Protection for Duplex stainless steels. Design includes many parameters like
- Current density.
- Coating breakdown factors.
- Mass of anode .
- Current output and many other.
- The article mainly involves the Cathodic Protection design for Duplex stainless steel pipeline.
- The Cathodic Protection design is also done for Carbon steel pipeline(48” dia with Al anode) and Duplex stainless steel( 24 “ dia with Zn anode) and are compared and analyzed with main objective, Duplex stainless steel pipeline (48” dia with Al anode).
- Finally the designs are interpreted, compared and analyzed.
What is Cathodic Protection:
- It is defined as “electrochemical protection by decreasing the corrosion potential to a level at which the corrosion rate of the metal is significantly reduced” (ISO 8044).
- “a technique to reduce corrosion of a metal surface by making that surface the cathode of an electrochemical cell” (NACE RP0176).
Need of Cathodic Protection:
- The cathodic protection is the one of the important factor in the performance of the pipeline carrying crude or petroleum products.
- If the pipeline has to be operated till its designed life with least maintainance activities, Cathodic Protection is the best way to adopt.
- The pipeline with a minimum care can lead to leaks, ruptures etc, which effects the supply and other contract terms, which results in loss of crude or product, demand, money, time etc..
- The main motto behind the Cathodic protection is “prevention is better than cure”.
- Its better to design and install a perfect cathodic protection rather opting a complex and un-predictable maintenance schedules.
Duplex stainless steel:
- The name itself implies that the steel is of two phase structure.
- The figure-1 shows the structure of Duplex steel.
- Commonly considered Duplex stainless steel is Austenite and Ferrite A.F region.
The different forms of Duplex stainless steel available are shown in Fig. 2. In this article DSS 22Cr 5.5 Ni 3 Mo 0.18N is considered.
Fig. 2: Different forms of DSS
ADVANTAGES OF DSS:
- High strength.
- A cost-effective alternative to austenitic stainless steels.
- High resistance to pitting, crevice corrosion resistance.
- High resistance to stress corrosion cracking, corrosion fatigue and erosion,
- Good sulfide stress corrosion resistance, except at high
- Low thermal expansion and higher heat conductivity than austenitic steels,
- Good workability and weldability,
- High energy absorption.
- Mainly used as subsea material.
- Cost factor.
- Fails at higher temperatures, which may result in Hydrogen Induced Stress corrosion Cracking(HISC).
- Need expertise welding. Otherwise it may lead to HISC, crevice corrosion, etc.
Design of Cathodic Protection:
- Pipeline diameter for Duplex with Al anodes, D = 48”.
- Length of the pipeline , LP = 71.650km.
- Pipeline joint length, Lj = 12m,
- Design life, T = 40 years,
- Design temperature, t =45 0 c.
- Corrosion coating thickness, tc = 5mm.
- Mean Coating breakdown factor, fcm= 0.05.
- Final Coating breakdown factor, fcf = 0.11.
- Gap between half shells, G = 100mm
- Anode spacing ,S = 4 joints.
- Electrochemical resistivity, for seawater, ξ = 20 ohm cm.
- Anode utilization factor, U = 0.8.
- Mean current density, i cm or CA = 80 mA/m2
- Final current densities, i cm or CF =150 mA/m2
Bracelet type of anode (Fig. 3):
For Duplex stainless steel pipeline the other parameters considered are:
- Al Anode material density, ρ= 2700 kg/m3.
- Electrochemical efficiency,ε= 1825 A-hr/kg.
- Protective potential, VP = – 500V.
- Closed circuit anode potential, VA = -1050 mV .
Design calculations :
- Diameter of the anode:
SL = S* Lj=48mm.
- Number of anodes,
N = LP / SL =1493.
- Net mass of each anode
MA= ᴨ/4((D+2tc)2– Di2)-2GtA)LAρ=196.2 kg
- Total anode mass
- Mean current output
IMC= εUM/T= 1219.692A
- Mean current required, IMR
= CAbAᴨDLF=80* 0.05*ᴨ* 1219*71650= 1097.56A.
- Final surface area per anode,
=ᴨ[1229+2(1-0.8)110-2*100]150= = 0.572m2 .
- Resistance of each anode,
- Final current output
IFC= N[VF – VA]/RA
- Final current required,
I FR = CFbFᴨ D LF
The below two parameters will say whether the whole calculation for CP is feasible or not.
- Mass requirement = IMC/IMR=692/1097.564= =1.11>1
- Final output current = IFC/IFR = 9893.373/4527.451 = 2.19>1.
The calculations are also done for
- Carbon steel pipeline with Al anode,48” dia.
- Duplex stainless steel pipeline with Al anode,48” dia.
- Duplex stainless steel pipeline with Zn anode,24” dia.
- From the calculations it can be inferred that the Duplex stainless steel requires less amount of anodic material.
- A graph (Fig. 4) is shown for Mass of the anode (kg) vs current output(A).
- It can be inferred that the Duplex with less amount of anodic material can give high current output, in case of Al anode.
- In case of Zn anode for Duplex stainless steel, more anodic material is needed for maintaining the current output, because the consumption rate of Zn is more.
Consumption Rate (Fig. 5):
- This graph shows the consumption rate of the anodic materials. Zn anode consumption rate is higher than Al. As a result more amount of material is need for meeting the current demand when compared to Al.
- Aluminium anode is much preferable because of its high capacity compared to other anodic materials. The graph below discusses about the
Capacities of anodic materials (Fig. 6):
Resistivity vs Current density:
From the graph below (Fig. 7), it can be inferred that the current may decrease along with the increase in resistivity.
- The focus is projected on the CP design for DSS and carbon steel with application of different anodes. The factors like mass of anode current output etc. are concerned for the performance analysis of CP with different anodes.
- It may be interpreted from the article that Duplex stainless steels with Al as anodes are best suitable for offshore applications.
- Hg free Aluminium sacrificial anodes are recommended for best performance of CP in subsea pipelines.
- Duplex stainless steels are best recommended for subsea pipelines because they are having good mechanical strength, high pitting corrosion and HISC resistant properties at normal temperatures.
- Highly skilled welding to avoid flaws in weld . The flaws may reasult HISC at the welded portion of the pipeline.
- Galvalum III are best recommended anodes certified by the DNV RP 401.It can even protect the hot oil pipelines which normal anodes may not do. These anodes are not susceptible to inter angular corrosion and can be used in place of Zn anodes.