Research on marine atmospheric corrosion resistance of ship plate steel

and high-speed ships and the development of offshore oil and gas fields provide broad application prospects for high-strength marine steel.
In order to ensure the safe navigation and reliability of the ship, the development of ship plate steel not only needs to make comprehensive considerations in terms of strength , toughness , fatigue performance and welding performance, but also puts forward higher requirements in terms of corrosion resistance .
NaCl salt content in the marine atmosphere is very large, and traditional carbon steel cannot form a protective rust layer. It is very necessary to develop a new type of ship plate steel that is resistant to marine atmospheric corrosion . At present, domestic research on the corrosion resistance of high-grade ship plate steel to marine atmosphere is relatively scarce .

The weekly immersion accelerated corrosion test avoids some shortcomings of the conventional atmospheric exposure test, such as long test period, large differences in regional conditions, volatile climate conditions, difficult to control, and unrepeatable tests . In this paper , the accelerated corrosion test of F690 , F460 and ordinary C-Mn steel Q235B was carried out, the corrosion resistance performance of different steel types was compared, and the characteristics of the corrosion products were studied .

Ship plate steels F690 and F460 are prepared by desulphurization treatment of molten iron , 180 t converter steelmaking , ladle refining ( LF ) , RH vacuum degassing and continuous casting to make 320 mm thick slabs . Ca is used to control the shape of inclusions during steelmaking, and light reduction is used in continuous casting to reduce the degree of central segregation of the slab . The chemical composition of the slabs is shown in Table 1 .

Table 1 The actual steelmaking chemical composition (mass fraction) of ship plate steel F690 , F460 and Q235B steel
Table 1 Chemical composition of the experimental steels ( mass fraction ) %

steel type	C	Si	mn	P	S	Nb	Cr	Cu	Ni	V	Mo	Fe
F690	0.07	0. 21	1. 03	0.008	0.002	0.042	0.6	1. 08	1. 68	-	0.47	margin
F460	0.07	0. 23	1.45	0.008	0.001	0.032	0. 15	0. 23	0. 22	0.041	-	margin
Q235B	0. 15	0.20	0. 68	0.02	0.015	-	0.02	0.04	0.02	-	-	margin

The continuous casting slab was equipped with a 5 000 mm four-high reversing rolling mill and MULPIC-
Controlled rolling and controlled cooling on wide and heavy plate production line with ACC accelerated cooling system
( TMCP ) process to obtain 50 mm and 60 mm thick steel plates , respectively . Quenching and tempering heat treatment of F690 steel plate: in 895 9
Temperature for 170 min, then quenched to room temperature, and then tempered, the tempering temperature was 650 °C , kept for 240 min , water cooled to room temperature after being out of the furnace; and the final state of F460 steel was TMCP state . Q235B steel plate is also a product obtained on the same hot rolling production line .

experiment method
Corrosion coupon samples are sampled close to the steel surface . The indoor accelerated corrosion test was carried out in a periodic immersion corrosion test chamber. The sample size of the periodic immersion test was 60 mm X 40 mm X 6 mm ( ± 0.5 mm ) , and the roughness of the planed surface reached 0.7 ^ m . Before the test, the sample was punched , cleaned with degreasing agent , dehydrated with absolute ethanol , degreased with acetone , weighed, and the mass was accurate to 0 . 000 1 g .
The cyclic immersion test is carried out in a dry-wet cyclic immersion corrosion tester . The test conditions are as follows: the immersion solution is 3.5% NaCl, the temperature of the water tank in the testing machine is ( 45 ± 2 ) C , the relative humidity in the box is 60% -80% RHo , and the cycle time is 60 minutes , and the immersion time is ( 12 ± 1.5 ) minutes .
Corrosion weight loss sample treatment: first mechanically remove the outer floating rust, and then immerse the sample in the rust removal solution ( 500 mLHCl+500mLH 2 O + 10 g C g HqN +4 g C 6 H 5 N 3 ) with a brush to remove corrosion products . Take 3~5 parallel samples for each group, whichever
Electro Probe Micro Analyzer ( EPMA ) -4610 in Japan was used to measure the enrichment of alloy elements in the rust layer and the cross-sectional morphology of the rust layer of the three steels after 30 days of corrosion. The resolution of the secondary electron image was 5 nm . Use the XRD ( X -ray diffraction device) Rigaku-D / max-2500 / PC of Rigaku , Japan, to analyze the composition of the rust layer after 30 days of corrosion of the sample , and perform semi-quantitative analysis, with a scan rate of 10° /min, 2 0 =10°~80° .
Table 2 Mechanical properties of F690 , F460 and Q235B steels
Table 2 Mechanical properties of F690 , F460
and Q235 B steels

steel type	Yield Strength R p0 . 2 / MPa	tensile strength Rm / MPa	Elongation A 5 / %	-60 seven impact absorption energy A kv /J
F690	782	813	20	176
F460	525	643	26	287
Q235B	250	320	twenty three	-

Table 3 Calculation results of annual corrosion thinning of test steel
Table 3 Calculated corrosion rates for the experimental steels

sample	Corrosion weight loss W t /	Sample length /	Specimen width /	Sample thickness /	Annual corrosion depth D /	relative corrosion rate /
serial number	g	mm	mm	mm	(mm • a -1 )	%
F690	10 . 01	59 ． 90	40 . 14	5 . 88	2 . 61	66 ． 00
F460	11 . 09	59 ． 76	40 . 06	5 . 96	2 . 90	72 . 00
Q235B	15 . 17	59 ． 39	40 . 06	5 . 99	3 . 98	100 ． 00

Table 4 Table 4	Quantitative volume percentage of corrosion products of F690 , F460 and Q235B steel after 30 days of corrosion ( % ) Fractions of the phase components in the rust layers of the experimental steels ( % )
steel number	a -FeOOH	0^eOOH	Y-FeOOH	Fe 3 O 4	other
F690	28. 2	9.2	14.3	48.2	0.1
F460	29.0	5.7	31.6	33.7	0
Q235B	19.0	11.0	'	70	0

in conclusion

• Ship plate steels F690 and F460 are in a state of uniform corrosion, with small surface bubbling, clear boundaries between inner and outer rust layers, and very dense inner rust layers . The surface of Q235B steel has obvious large bubbling, obvious pitting pits, and no obvious boundaries between inner and outer rust layers. And all are relatively loose and cracks are observed . The corrosion rate of F690 is significantly lower than that of ordinary C-Mn steel .
• Cr and Mo elements in the rust layer of ship plate steel F690 , forming a relatively dense protective rust layer; there is a boundary between the inner and outer rust layers of F460 , and it is determined that there are alloy elements Cr rich in the pitting pits of the inner rust layer. set .

( 3 ) The rust layer of ship plate steel F460 and F690 is composed of a -FeOOH , 0-FeOOH , Y-FeOOH and Fe 3 O 4 Composition, the content of phase a-Cr x Fe i_x OOH in the rust layer of F690 steel is more, Mo is distributed in the cracks of the inner rust layer, making the inner rust layer more dense, and this rust layer can obviously improve the corrosion resistance of steel .

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