Effect of Alloying Elements and Carbon Content on Corrosion Behavior of E36 Ship Plate Steel

E36 grade low alloy ship plate steel in high concentration C1- environment was studied by weight loss method, scanning electron microscope, polarization curve, electrochemical impedance spectroscopy ( EIS) ; Effect of E36 steel on corrosion resistance in 10% NaCl acidic solution.

The results show that the corrosion rate of ultra-low carbon steel ( 0.048 %) is lower than that of low carbon steel (0.1 % ); the corrosion rate of L steel with added 辂 element is lower than that of 2# steel without lead , but the addition of 辂 element At the same time, the steel with higher carbon content has the highest corrosion rate. This shows that in the low-saw range, the improvement of corrosion resistance by adding saw elements is not as obvious as that of reducing carbon elements.

The corrosion of crude oil itself and the special state of oil tanker transportation are the most important factors causing corrosion of crude oil cargo tanks ( COT for short) . Corrosion of cargo oil tanks may cause breakage of ships or crude oil leakage accidents of oil tankers, which not only cause huge economic losses, but also cause serious harm to the marine environment and marine life. The amount of steel used in cargo oil tanks of crude oil tankers accounts for 30% to 40% of the total amount of steel used in oil tankers. Judging from the annual steel consumption of oil tankers in China, the amount of steel used in cargo oil tanks should be about 2 million tons per year . In the past, the treatment of cargo oil tanks was to coat steel plates, but such coatings can only last for about 10 years in harsh corrosive environments . Maintenance, which not only increases the cost, but also cannot accurately predict its safety.
Bottom plate corrosion is mainly caused by the seawater mixed in the crude oil, which is usually caused by water injection during oil production. In addition, the condensed water formed due to the alternation of temperature also enters the crude oil, and after long-term water submersion and oil flotation, a layer of sedimentary seawater containing high concentration of NaCl is gradually formed at the bottom of the cargo oil tank. Research by the Japan Shipping Committee shows that:

① there is about 10% (wt) NaCl water solubility in crude oil ;

② the pH in corrosion pits is lower than 1.5 ; . Domestic studies have also been carried out on the environment of the lower floor of the cargo oil tank .

1 test
The composition of the test materials is shown in Table 1, and steel ingots ( 1# and 2#) with different carbon contents (1# and 3* ) and different carbon contents were designed . The steel ingot is smelted by a 25kg vacuum induction furnace. After heating and forging, a 90 mmX90 mmX 90 mm forged billet is subjected to two-stage controlled rolling on a test rolling mill. The rolling process is as follows: the slab is heated to 1 200 -C, and then rolled at 1 100 C. , rolled for 3 passes, and the thickness reached 45 mm ; in the second stage , rolling was started at 860 °C , rolled for 4 passes , and final rolling was carried out at 800 C , with a final rolling thickness of 15 mm . It is ferrite + a small amount of pearlite.

Table 1 The chemical composition of the steel used in the test

steel number	C	Si Mn		P	the s	Nb	Ti	Mo	Cu	Ni	Cr	Ce
p	0.048	0.35	1.49	0.004	0.0027	0.025	0.015	0.022	0.34	0.34	0.24	0.085
2#	0.048	0. 35	1.42	0.004	0.0042	0.025	0.015	0.020	0. 33	0.3	-	0.12
3 recommended	0.1	0.37	1.43	0.004	0.0031	0.025	0.015	0.021	0. 34	0.32	0.23	0.1

Accelerated corrosion samples were prepared with a size of 60 mmX25 mmX 5 mm . Grind with 600# sandpaper, dehydrate with absolute ethanol and degrease with acetone before corrosion, and weigh it as Wo . Accelerated corrosion test uses NaCl solution with a mass fraction of 10% , adding HC1 dropwise to adjust the pH of the solution to 0.85 o . Corrosion solution, the corrosion test time is 72 h . After the test, three parallel samples were taken for each steel type , and after the parallel samples were taken out, they were pickled and derusted with corrosion inhibitor (1000 mL HC1, hexamethylaniline) at room temperature, and then cleaned with absolute ethanol. Then it was dried and weighed as Wi, and the corrosion rate was calculated by the weight loss method.

mechanical properties
The mechanical properties of the tested steel after controlled rolling and controlled cooling are shown in Table 2 .
NV stands for Det Norske Veritas.
Table 2 Test steel mechanical properties

steel	Yield Strength	tensile strength	Elongation	-40"C low temperature
Number	/MPa	/MPa	/%	Impact energy /J
NV	>355	470 ~ 620	twenty one	twenty four
P	460	580	32	60.7
2#	473	565	36	60
3 turns	495	595	33	49

Through the solid solution strengthening and precipitation strengthening of alloying elements, the carbon content of Yangang is 0.048%, and the yield strength reaches 460 MPa after water cooling ; the carbon content of 3# steel is 0.1 %, and the yield strength reaches 495 MPa after water cooling , but the impact toughness is poor , 1 # and 2 # after water cooling, the impact toughness difference is not big. It can be seen that due to the higher carbon content of 3# , the yield strength is higher and the low temperature impact toughness is poor.
Corrosion rate of E36 ship plate steel
Calculate the corrosion rate of the sample according to formula 1 , and the average corrosion rate of the three test steels

The average corrosion rate is shown in Table 3 .
= 365 X 24 X Aw X 10 ⑴
_ SX72X P _
In the formula: S is the surface area of the steel ( cn?) and is the density of the sample ( g cm -3 ) ; Aw is the mass loss before and after corrosion (g) o
seen from Table 3 that the corrosion rate of 3# steel is much higher than that of 1# and 2#, and the corrosion rate of the three steels from high to low is : 3 #>2#>1#. Corrosion in concentrated CT environment is detrimental

effect.
Table 3	Average corrosion rate of test steel
steel number	Average corrosion rate /(mm・a -1 )
]#	0. 58
2#	0. 74
3#	2.5

E36 ship plate steel conclusion

• the corrosion resistance of E36 steel in low pH and high concentration CT at ultra-low carbon content . The steel with the same sawing content and higher carbon content will form more carbides with carbon, which will become the cathode for pitting corrosion, and the number of pitting nucleation will increase, resulting in severe localized corrosion.
• corrosion rate of 3# steel, which added element and increased carbon content at the same time , was greater than that of 2 # steel which only reduced carbon content without adding saw . It shows that in the low sawing range, the influence of carbon content on corrosion resistance is greater than that of sawing element content on corrosion resistance. This is due to the increase in carbon content, more saw and carbon form carbides, resulting in a decrease in the amount of solid solution saw, and thus a decrease in the amount of protective Cr 2 O 3 .

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