TITEL
HIGH PHOSPHORUS PIG IRON AS SACRIFICIAL ANODE FOR CATHODIC PROTECTION OF UNDERGROUND MILD STEEL STRUCTURES
FIELD OF THE DISCLOSURE
The present disclosure relates to a metallurgical pig iron.The disclosure further relates sacrificial anode for cathodic protection of underground steel pipelines.
BACKGROUND OF THE DISCLOSURE
Conventionally zinc, aluminium and magnesium based alloys are used as to provided sacrificial anode to underground pipelines.
When the zinc based anodes are used, they tend to contaminate the local water bodies by solubilized zinc which may cause ecological damage and can also contaminate the food chain if the zinc level in water rises beyond 5ug/L.
When the aluminium based anodes are used, they tend to form passive oxide film on pure and unalloyed aluminium (A.H. Al-Saffar et at.). This restricts their use as sacrificial anodes. Hence, in order to make it suitable for cathodic protection they are often alloyed with mercury (Hg), indium (In), tin (Sn) and titanium (Ti). However, aluminium sacrificial anode is only limited to marine application.
Magnesium based anodes have low current efficiency of about 50% which have been reported to be due to enhanced corrosion rate of the anode. As a result, in order to increase the efficiency of magnesium sacrificial anodes, they are alloyed with aluminium, zinc and manganese that add up to the costHence, use of pig iron as sacrificial anode for cathodic protection could be more economical due to its mass production and environmental friendly.
Further, the cathodic protection based on Mg, Zn and Al alloys are environmentally hazardous and they tend to contaminate the environment (Jonathan Deborde et al.). Moreover, Al, Zn and Mg are costly metals.

OBJECTS OF THE DISCLOSURE
In view of the foregoing limitations inherent in the prior-art, it is an object of the disclosure to propose a suitable material that may give the cathodic protection to the underground pipelines.
The other object of the disclosure is to propose a suitable material that is comparatively cheap to conventionally used material for such applications.
The other object of the disclosure is to propose a suitable material that is environment friendly while providing the cathodic protection.
SUMMARY OF THE DISCLOSURE
The disclosure provides a pig iron / cast iron comprising phosphorus 1.1-2.5 wt.% carbon 3.1-4.2 wt%, manganese 0.45-0.95 wt.%, silicon 1.5-2.5 wt.%, sulphur 0.025-0.04 wt.%, inevitable impurities and Fe as balance, and blow holes generated over surface of the said pig iron/cast iron.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figs, la & lb illustrates secondary electron SEM micrograph of the fracture surface of pig iron and corresponding EDS area mapping respectively in accordance with one of the embodiment of the disclosure.
Figs. 2a & 2b illustrates secondary electron SEM micrograph of the fracture surface of pig iron and corresponding EDS area mapping respectively of area other than that of Figs, la and lb in accordance with one of the embodiment of the disclosure.
Fig. 3 illustrates back scattered electron (BSE) SEM micrograph of pig iron showing elongated entrapped inclusion in accordance with one of the embodiment of the disclosure.
Fig. 4 shows inclusion in the pig iron in accordance with one of the embodiment of the disclosure.

Fig. 5 shows experimental setup for real time potential and current measurement in accordance with one of the embodiment of the disclosure.
Fig. 6 shows surface appearance of the mild steel and pig iron after exposing the couple for a month in soil in accordance with one of the embodiment of the disclosure.
Fig. 7 shows surface appearance of the mild steel and Mg rod after exposing the couple for a month in soil in accordance with one of the embodiment of the disclosure.
Fig. 8 shows OCP variation for mild steel and pig iron in accordance with one of the embodiment of the disclosure.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE DISCLOSURE
Various embodiments of the disclosure provide A pig iron /cast iron comprising phosphorus 1.1-2.5 wt.%, carbon 3.1-4.2 wt.%, manganese 0.45-0.95 wt.%, silicon 1.5-2.5 wt.%, sulphur 0.025-0.04 wt.%, inevitable impurities and Fe as balance, and blow holes generated over surface of the said pig iron/cast iron.
The said composition of the pig iron/cast iron can be obtained conventionally through blast furnace route or through electric arc furnace. The molten metal obtained through the said route can be further refined as per the composition defined.
Composition of the Pig iron
The high phosphorus content (1.1-2.5 wt %) for the said pig iron is maintained so as to make it behave as a sacrificial anode for cathodic protection of mild steel due to enrichment of the surface with phosphorus as iron phosphate hydrate (FeP04.2H20) under immersed condition. The layer thus formed is highly unstable in nature and undergoes continuous dissolution in the soil submerged condition that could help in supplying electrons required for cathodic reaction on the surface of mild steel, which is to be protected cathodically. The composition of the carbon is maintained at 3.1-4.2 wt%.
In accordance with a specific embodiment of the disclosure for explanatory purpose, Pig iron is produced with the following composition as shown in Table 1


Manganese (0.45-0.95 wt%) and sulphur (0.025-0.04 wt%) are present in pig iron. Shown in Figs, la & lb and Figs. 2a & 2b is the pig iron produced as per the composition mentioned in Table 1 depicting the fracture surface and subsequent EDS area mapping. Segregation of Mn, S and P are also shown along the broken ligaments. The fracture surface shows typical brittle faceted fracture. It suggests that these ligaments are very active regions since Mn along with S would act as anodic regions, allowing pitting to initiate along these zones. It has also been reported that presence of MnS inclusion plays a leading role in enhancing the initial corrosion rate in chloride containing environment as CI ions get adsorbed and accumulated around MnS inclusions resulting in localized corrosion. Hence, here also enriched S and Mn ligaments would behave similarly. Moreover, P enrichment along these ligaments also accelerates the corrosion process.
Table 2 shows the composition of the entrapped inclusion on pig iron based on the composition mentioned in Table 1.

Fig. 3 and associated EDS composition (Table 2) does indicate that these greying regions are richer in S. However, Matrix contains little S.
Presence of silicon (1.5-2.5 wt %) results in graphrtization and forms graphite flakes in the pig iron matrix. Also presence of Si results in formation of inclusions of Si02 along with unreduced FeO. However, effect of silicon is more on the graphitization, as seen in the present pig iron. Low silicon content is beneficial from the point of anodic nature of the pig iron since low content of Si does not allow any protective layer formation.

Pig iron has a phosphorus content of 1.1-2.5 wt%. Under immersed condition in soil, the initial corrosion rate is high due to enrichment of the surface with higher phosphorus content at the metal/scale interface which eventually results in formation of ionic phosphate at the surface. Based on thermodynamics, the free energy for the formation of FeP04 and and 1 bar pressure are,

Hence, the formation of iron phosphate is much favoured at the rust/metal interface which results in further corrosion of the surface.
The said pig iron /cast iron generate blow holes on its surfaces. This presence of blow holes is a result of impurities and entrapped inclusions which may also accelerate the localized attack due to the presence of chloride ions present in the soil and enhance the corrosion rate.
The entrapped inclusions observed in the matrix of pig iron are surrounded by higher volume fraction of pearlite. Under completely immersed condition, these entrapped inclusions would act as cathodic with respect to the iron matrix as they are enclosed with higher volume fraction of peariite as shown in Fig. 4. This results in higher corrosion rate of iron matrix under complete immersed condition. These entrapped inclusions are also responsible for the breakdown of the protective passive film on the surface of pig iron under complete immersed condition. The interfaces between the iron matrix and the entrapped inclusions have also been found to behave as an ideal potential site for pitting corrosion.
The Driving voltage (V) for sacrificial behaviour of the obtained pig iron / cast iron with respect to mild steel in soil atmosphere is (0.04 - 0.07) V.
The electrochemical capacity of the pig iron / cast iron in soil atmosphere is (100 - 110) Ah/Kg.
The anode to soil resistance for the pig iron / cast iron acting as an anode is (480 - 500) Ohm.+

Further, the obtained pig iron has the corrosion rate when coupled to mild steel in soil atmosphere is
Advantages:
The obtained pig iron provides the cathodic protection to the underground pipelines.
Further, the cost of pig iron is much cheaper as compared to the available sacrificial anodes in the market as shown in Table 3
The obtained pig iron is environment friendly while providing the cathodic protection.

Based on the above cost analysis shown in Table 3, it is evident that the Pig iron is quite comparatively cheap.
Experimental Analysis
The disclosure is focused on use of pig iron as sacrificial anode for cathodic protection of underground steel pipelines. Cathodic protection is based on the principle of supplying electrons to the metal structure to be protected. Sacrificial anode type cathodic protection system provides cathodic current by galvanic corrosion. Current is generated by metallically connecting the structure to be protected to a metal/alloy that is electrochemically more active than the structure to be protected.
An experiment was performed to analyse the performance of pig iron as sacrificial anode during cathodic protection of steel plates.It was then compared with magnesium rod that was also used as sacrificial anode to cathodically protect the same composition of steel plates.

The dimensions of the various materials used in the experiment is as shown below-

The chemical compositions of the various materials used (in wt.%) in the experiment is shown below
Mild steel with Pig iron

Magnesium anode composition Mg-99.86 Fe-0.13 Si-0.01
In the first setup, pig iron was connected to the steel plate, which was to be protected, using a copper wire.
In the second setup, the magnesium rod was connected to the similar steel plate of same dimension.
The copper wire at the point of contact with the steel plate and the sacrificial electrode was carefully insulated using lacquering. The insulation was done in order to prevent the galvanic coupling that could form between steel plate and the copper wire resulting in the corrosion of steel plate. Another purpose of insulation was to prevent any current leakage.

Both pig iron and magnesium anodes were surrounded by backfill. The purpose of backfill is to improve the electrical contact between the sacrificial anode and the surrounding soil. The composition of backfill used is:

Both the setups were buried under the soil separately for 1 month. They were kept at appreciable distance in order to ensure no stray current effect between the two setups. Potential and current were measured at real time with the help of program installed on Arduino Uno software.
Composition of soil (shown in Table 4) was analysed using ion chromatography. The pH, moisture content and resistivity of the soil were determined based on ASTM G51-95, ASTM D4959-16 and ASTM G57-06 respectively.

After the completion of 1 month of experiment, the rust was cleaned using the following chemicals based on ASTM G1-03.

The experimental setup for real time potential and current measurement is shown in Fig. 5.

After one month of experiment the samples were taken out from the soil. It can be seen that in case of galvanically coupled pig iron and mild steel (Fig. 6), the surface of pig iron appears to be more corroded as compared to mild steel.The surface of the steel plate faced towards the pig iron anode was not corroded much and similar behaviour was seen in case of steel plate connected to the magnesium rod with still better appearance. However, as compared to pig iron, magnesium anode was aggressively corroded (Fig. 7) which signifies that pig iron can be used for longer period.
The rust formed on the surface of the pig iron under the soil has been characterized which confirms the absence of magnetite (Fe3O4). This implies that the rust thus formed on the surface of pig iron is not protective.
Thus, based on the above experiments it can be concluded that pig iron could be a potential candidate as a sacrificial anode during cathodic protection of steel plates and there could be a scope to use it industrially since it will be much cheaper options as compared to magnesium.
It can also be proven from the electrochemical experimental analysis that Pig iron is more active as compared to mild steel.
The electrochemical tests were carried out in Parstat 2263 system in a Flat bottom cell consisting of a specimen, a saturated calomel electrode with saturated calomel (+0.2444 V versus SHE) and platinum wire mesh as the working, reference and counter electrodes, respectively.
In a 3.5% NaCI solution after 1 hr of stabilization, the OCP for pig iron is (-0.680 V) and mild steel is (-0.655 V) (shown in fig. 8).
Formulas used during the calculation are given as follows;
The output current is given by ohm's law


The resistivity of the soil was computed using Wenner four pin method used in laboratory scale with probe spacing of (a=0.25 cm). However, the instrument used in field measurement has a minimum probe spacing of (A=76.2 cm). So, we introduce a proportionality constant K (=A/a) to the resistivity to replicate the field resistivity condition. Hence, the resistivity of the soil is calculated according to the formulae,

Where,
K is the proportionality constant (=305)
R is the soil resistance measured from the Instrument (=20.01 £2)
The total resistance can be approximated using H. B. Dwights equation which is given as:

However, for anode to cathode separation of less than 30 cm, a correction factor of 1.3 is introduced to the total resistance (DNV-RP-B401).
Where,

The practical electrochemical capacity (Ah/Kg) can be calculated as:

Where



Note 1- the driving voltage is the potential difference between the sacrificial anode and steel plate when buried under the soil which was measured with the help of Arduino Uno software.
Note 2- in order to calculate the anode to soil resistance, the dimensions of the anode including the backfill are: For Mg anode: L = 0.1574 ft, D = 0.1476 ft and For Pig iron anode: L = 0.1640 ft, D = 0.1377 ft.
Note 3: the corrosion rate was determined using the following formula-



Thus, the above experiment shows that pig iron remains anodic to mild steel and it didnt switched to cathode during the entire one month period of the experiment. So it can be used as sacrificial anode for cathodic protection of mild steel.
In order to achieve similar protection of steel plates as achieved by magnesium, the mass of pig iron required is 10 times more than that of magnesium along with increase in the number of anodes by maintaining similar mass. This increase in number of anodes will add up the current value that will be supplied to the steel plate. But even than the cost would be nearly 4-5 times lower than that of magnesium anodes. Hence, pig iron sacrificial anode is cost effective as compared to magnesium sacrificial anode.

WE CLAIM:
1. A pig iron /cast iron comprising:

inevitable impurities and Fe as balance, and blow holes generated over surface of the said pig iron/cast iron.
2. The pig iron/cast iron as claimed in claim 1, wherein the driving voltage (V) for sacrificial behaviour of the pig iron with respect to mild steel in soil atmosphere is (0.04 - 0.07) V.
3. The pig iron/cast iron as claimed in claim 1, wherein the corrosion rate of the pig iron when coupled to mild steel in soil atmosphere is (0.25 - 0.40) mm/year.
4. The pig iron/cast iron as claimed in claim 1, wherein the electrochemical capacity of the pig iron in soil atmosphere is (100 -110) Ah/Kg.
5. The pig iron/cast iron as claimed in claim 1, wherein anode to soil resistance for the pig iron acting as an anode is (480 - 500) Ohm.