Deep-sea environment is very harsh, and will have a huge impact on underwater vehicle and deep-sea device. This article discusses of the corrosion related factors in deep-sea environment such as the dissolved oxygen, pressure, salinity, temperature, flow rate etc. as well as their impact on the corrosion of metals, alloys and other materials, then comes to the conclusion that the dissolved oxygen is the most important factor for the corrosion of metals and alloys. The research status quo of deep-sea environment corrosion of metal and alloy materials are reviewed with emphasis on four common types of corrosion such as pitting corrosion, crevice corrosion, tunnel corrosion and stress corrosion. Finally countermeasures for the corrosion control of metals and alloys in deep-sea environment are also introduced.
The electrochemical behavior of X80 pipeline steel in simulated red soil solutions with different pH, which aim to simulate the corrosive medium of typical red soil at Yingtan area of the Southeast China was studied by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and three-dimensional video microscope. The results show that the corrosion of X80 steel in simulated red soil solution is controlled by oxygen depolarization reactions when pH is 5.5; a corrosion products scale formed on the steel surface due to corrosion; the EIS of the corroded steel is composed of incomplete capacitive arc at high-frequency and complete capacitive arc at low-frequency. With increasing pH value, the corrosion processes of X80 steel gradually convert into being controlled by electrochemical activation, while its corrosion current density increases gradually and its corrosion resistance decreases. When pH value reduces to the range 4.0~3.0, numerous corrosion pits occur on the surface of X80 steel and correspondingly inductive arc appears in the low-frequency region of EIS.
Electrochemical behavior of galvanized Q235 steel for grounding in acidified soils with different pH value was investigated utilizing electrochemical impedance spectrum (EIS) and potentiodynamic polarization tests. Results showed that the corrosion behavior of galvanized Q235 steel in acid soils presents an adsorption of intermediate corrosion products in the early stage, and a low frequency inductive loop then induced in Nyquist plot. With the formation of corrosion products on the surface of the steel, the inductive loop disappeared, the low frequency capacitive arc enlarged gradually. The corrosion process is mainly controlled by the cathodic process. The corrosion rate of the galvanized Q235 steel decreased with the increasing pH of the acidified solution, it also increased and then decreased with the increasing time.
The corrosion behavior of X100 pipeline steel in an artificial solution which aims to simulate the alkaline soil medium at Korla area of Xinjiang were investigated by weight-loss, electrochemical measurement, scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and X-ray diffractomater (XRD). Results show that the corrosion behavior of X100 pipeline steel in the simulated solution is a typical active dissolution corrosion. The corrosion tendency gradually increases with time. The average corrosion rate and the instantaneous corrosion rate all increase first, then decrease and increase again finally. The corrosion products exhibit a three layered structure composed of a surface layer of loosened brown red γ-FeOOH, a middle layer of black loosened Fe3O4 and FeS and an inner layer of dense black Fe3O4 and FeS. It is also found that Cl- plays an important role in X100 steel corrosion, and a small amount of SO42- is also involved in the reaction which has certain influence on the corrosion.
Effect of pH values on pitting dynamics and morphology of single pits of 304 stainless steel in 3.5%NaCl solution was studied by means of potentiostatic polarization and 3-dimensional video microscope. The results showed that the pitting current, the growth rate of pit volume, the pit depth and pit mouth diameter of single pits increase with the decreasing pH in a range 1 to 14 by applied potential of 0.15 Vvs SCE. In solutions with the same pH, the growth rate of pit depth increases with time, while the growth rate of pit mouth diameter decreases with time. In solutions with different pH, the growth of single pit is controlled by the diffusion through corrosion products. There exists lacy cover on the wall of pit mouth, and the integrity of the lacy cover declines with the increase of pH.
Effect of crude oil properties on the corrosion behavior of 16Mn steel was evaluated by means of grey relationship analysis method. The mass of the effect of each corrosive factor related with the main properties of crude oils on the corrosion rate of 16Mn steel was analyzed by hierarchy process. Results showed that the mass of corrosive factors of the crude oil could be accessed as:salinity (0.51), sulphur content (0.264), nitrogen content (0.129), water content (0.064) and acid value (0.033) respectively. Then the corrosivity of crude oils could be evaluated by comprehensive analysis of the corrosive factors and their weight: correspondingly the corrosivity of crude oils from 6 selected regions could be ranked with a descending sequence as follows: Liao River>Palanca>Huizhou, Jinzhou>Daqing>Shenbei.
Two new benzimidazole derivatives, BSC6T and BSC8T, were synthesized using 2-aminobenzimidazole and triazine anionic surfactants. Their inhibition effect on steel 45(GB) in 1 mol/L HCl solution were studied by means of mass loss method, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Results showed that the inhibition efficiency increased significantly with the increasing inhibitor concentration; BSC6T and BSC8T acted as a mixed type inhibitors with cathodic inhibition as the dominative action; BSC6T and BSC8T had better inhibition effect rather than 2-aminobenzimidazole; however, among the three inhibitors, BSC8T exhibited the best inhibition performance with inhibition efficiency up to 98.6% for a concentration of 0.9 g/L.
Poly N-vinylimidazole (PVI) was synthesized using N-vinyl imidazole as monomer and azodiisobutyronitrile (AIBN) as initiator in a solvent of benzene. Its corrosion inhibition for Q235 carbon steel in HCl solution were investigated by means of mass loss method, Tafel polarization measurement and electrochemical impedance spectroscopy (EIS). Results indicate that inhibition efficiency of PVI can reach 90.1% in 1 molL-1 hydrochloride acid with 2 mgL-1 PVI at 298 K, and it can reach above 84% at high temperature (328 K) in solution of high acidity (2 molL-1). The electrochemical measurements suggest that PVI is a kind of mixed type inhibitor, the adsorption of PVI on the surface of metal obeys the Langmuir adsorption isotherm, free energies of adsorption calculated indicate that PVI exhibits strong tendency of adsorption on metal via chemical adsorption.
Tests of alternate corrosion and fatigue for 2A12-T4 aluminum alloy were performed in order to reveal the influence of factors related with corrosion environment and load on its fatigue life. The effect of the alternate frequency of corrosion/fatigue and the type of corrosion tests such as alternate immersion, salt spray and salt spray+hot and humidity corrosion on the degradation of 2A12-T4 aluminum alloy was analyzed. The results show that for a designated total time of corrosion, the alternate frequency of corrosion/fatigue has important influence on the fatigue life of the alloy. The fatigue life of the alloy increases gradually with the increasing alternate frequency. The total fatigue life of aluminum alloy for 2, 4 and 6 cycles were 16.9%, 30.7% and 50.3% respectively higher than that for the first cycle. Besides, the type of corrosion tests also has important influence on the fatigue life of the alloy, of which the impact intensity may be ranked in a descending order as follows: salt spray corrosion>alternate immersion corrosion>salt spray+hot and humidity corrosion.
The current efficiency and corrosion mechanism of Al-Zn-In sacrificial anode by different impressed current densities were investigated by using electrochemical methods. Results showed that the corrosion mechanism and the current efficiency of the Al-Zn-In sacrificial anode varied with the impressed current densities. The current efficiency decreases dramatically by a low current density, this may be caused by the fact that the re-deposition of the dissolved cations (In3+, Zn2+) was suppressed and an oxides film might formed on the alloy surface. A mass loss of the anode might be caused by the cracking and dissolving of the oxides film, and this might be responsible to the current efficiency of Al-Zn-In sacrificial anode lower than that theoretically expected by the low impressed current density.
Corrosion behavior of three titanium alloys (TC4, TC18, TC21) in 3.5%NaCl solution was studied by means of cyclic voltammetry, SCC test with WOL pre-cracked sample and scanning electron microscope. Results showed that the pitting corrosion potential Eb of the three titanium alloys were 1.625, 1.671 and 1.871 V for TC4, TC18 and TC21, respectively. All values of Eb were high, representing the excellent pitting corrosion resistance of them. The pitting corrosion susceptibility of them could be ranked as the following descending sequence: TC4>TC18>TC21. The KISCC for TC4, TC18 and TC21 were 62.92, 66.82 and 71.99 MPam0.5 respectively and the SCC susceptibility of the three titanium alloys followed a sequence as: TC4>TC18>TC21. On the fractured surface of the alloys after SCC test, a macro-morphology with three zones representing pre-crack, stress corrosion induced crack and mechanical fracture respectively were clearly differentiated, while the stress corrosion cracking zone showed mainly ductile fracture.
Titanium based PbO2 electrode was surface modified by electrodepositing in a polyvinyl alcohol (PVA) containing electrolyte, then its phase composition and microstructure was characterized by SEM and XRD. While the electrochemical properties of the modified electrode was examined by means of electrochemical impedance spectroscopy (EIS), linear potential sweep (LPV), degradation experiment of phenol and accelerated lifetime tests. Results show that the modified electrode has a high-dense structure composed of network-like PVA on mesh-like PbO2. The surface modification process also results in decreasing the particle size and increasing the specific surface area of the film on electrode. Compared with the original PbO2 electrode, the modified PbO2 electrode presents a relatively lower electrochemical impedance as well as a higher oxygen evolution reaction potential. Finally, the modified PbO2 electrode exhibited higher electrocatalytic activity and longer service time. As an example, 96.9% of phenol in artificial waste water can be removed within 2 h by an electrocatalytic degradation process with a properly modified electrode PbO2-PVA (0.6) and the service time of that electrode was ca 1.5 fold of the original one.
Galvanic corrosion behavior of couples of cold rolled steel SS400 and hot rolled steel ST12 with chrome-free Zn/Al coating or Zn/Ni alloy electroplating was studied in 3.5%NaCl (mass fraction) solution by means of potentiodynamic polarization and galvanic method. The results showed that the couple of SS400 and ST12 with Zn/Ni alloy electroplating exhibited a weaker tendency of galvanic corrosion, a higher open circuit potential and a smaller galvanic current rather than that with chrome-free Zn/Al coating. While the free-corrosion current of Zn/Ni alloy electroplating coating was smaller than that of Zn/Al coating. Therefore, Zn/Ni alloy electroplating is extremely promising coating for the auto-mobile fastener to replace the traditional Zn electroplating.
