Study of corrosion behavior of labeling medium on Fe-36%Ni cold-rolled strip use

,

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: To fulfill the high-quality surface requirements of the Fe-36%Ni cold-rolled strip used in liquid natural gas carriers,corrosion tests were conducted on alloy surfaces using ink media with different composition in high-temperature and high-humidity conditions.The results show that the Cl- content in ink is the main cause of surface corrosion of Fe-36%Ni alloy at 95% RH and 50 ℃.Cl- penetrates the passivation film,causing pitting on the surface.This corrosive material primarily comprises elements Fe and O.After 120 h of high temperature and high humidity,pitting had occurred on the surface,which was characterized by irregularly distributed areas of bright red filamentous corrosion.With time,the corrosion gradually deepened and expanded to nearby areas.However,when the ink medium contained no Cl-,no corrosion was found on the alloy surface.As such,during the production of Fe-36%Ni material and its application in LNG shipbuilding,care must be taken in the selection of the marker or medium that will come into contact with the strip surface to ensure that it contains no corrosive ions like Cl-.

Key words: corrosion; Cl-; Fe-36%Ni; high temperature and high humidity; LNG

1 Introduction

Fe-36%Ni is a nickel-iron-based alloy with a Ni con-tent of 36%.Due to the abnormal magnetic telescopic effect,this alloy has a very low expansion coef-ficient both at ambient and cryogenic temperatures.At -196 ℃,the alloy’s yield strength exceeds 900 MPa with good resilience.It is widely used in the construction of membranes for liquid natural gas(LNG) ships.When 0.7-mm-thick strip is in direct contact with LNG,the surface of the strip must be of high quality and have no contaminant residue,the presence of which promotes membrane-surface cor-rosion that can result in natural gas leakage.These strips are frequently marked with markers during the manufacturing process.Due to differences in the composition of the ink in the marker,areas marked by these pens may rust under certain atmospheric environments.The correct choice of ink in the mar-ker has an important impact on the manufacturing process or product application.At present,there is scant research on the corrosion behavior of Fe-36%Ni alloy.The aim of this paper is to consider the corrosion effects of different inks on Fe-36%Ni alloy surface in high-temperature and high-humidity environments and to analyze the corrosion mech-anism.

2 Test method

This test was conducted in a Japanese Espec PL-3KP constant-temperature-and-humidity chamber that can accurately control temperature and humi-dity to simulate atmospheric changes.The Fe-36%Ni strip was manufactured using an electric-furnace and continuous-casting process followed by hot rolling and cold rolling to a thickness of 0.7 mm.Table 1 lists the typical chemical compos-ition of the strips.In this study,several different composition ink media were used to mark the strip.The marked sample was placed on a glass stand rather than a metal stand to avoid electrochemical corrosion in the environment testing chamber.The humidity and temperature were set to 95% RH and 50 ℃.The holding time was 1 000 h and samples were taken out of the chamber for examination at 120,250,and 500 h.

Table 1 Typical chemical composition of Fe-36%Ni for testing %

3 Results and analysis

3.1 Comparisons of strip corrosion from dif-ferent ink composition

Fig.1 shows the surface corrosion morphology of Fe-36%Ni alloy after 1 000 h in the environmental testing chamber.Fig.1(a) shows scanning electron microscopy(SEM) images of the macro-mor-phology of the sample surface and Figs.1(b)-(d) show SEM images of the micro-morphologies of three samples with different ink composition.The edge of the ink imprint and the rolling direction are clearly observable in Figs.1(b) and(c).Although it is clear that no corrosion occurred in samples 1#and 2#,bright red corrosion areas are visible in the marked area of sample 3#in Fig.1(d).The SEM image shows a corrosion microstructure charac-terized by dot-shaped or irregular filaments,some of which had even turned to powder,as shown in Fig.1(d).It is clear that corrosion covers all the marked areas,whereas unmarked areas remain clean and bright.From these results,it can be concluded that the composition of the ink is the main factor causing surface corrosion of Fe-36%Ni alloy.

Marker ink is generally an organic solvent consisting mainly of carbon and hydrogen along with some other chemical elements.To compare the differences among three inks,SEM chemical analysis was conducted on an original marked surface that had not been in the environmental testing chamber.The results show that the composition of the sample surfaces after having been marked is basically similar.Except for the matrix elements Ni and Fe,the main surface element is C,which accounts for about 40% of the total mass fraction,making it the main constituent element of the organic solvent in the ink.O accounts for about 4% of the total mass fraction.However,the 3#sample differs from the other two in that it contains about 0.37% Cl.The results are listed in Table 2.

Table 2 Composition of original marked sample surfaces with no holding time %

Fig.2 clearly shows the morphology of the corrosion pits of sample 3#,which are shallow circular pits on the surface of the alloy.Some of the corroded areas have fallen off and left the alloy matrix exposed.Table 3 shows the results of the SEM chemical analyses of the corrosion and exposed matrix surfaces.In addition,Cl is detected in the corrosion material and accounts for about 1.65% of the content,which is more than four times the 0.37% content of the original marked surface.As an acidic anion,the Cl-has a small atomic radius and good diffusion ability,which means it can easily attach to a variety of metal surfaces,resulting in surface passivation film damage and pitting[1-3].As a fully austenite alloy,the 36% Ni plays an important role in stabilizing the austenite structure and preventing the transform-ation of austenite into martensite at cryogenic tem-perature(-196 ℃).It also has a stable minimum coefficient of the linear expansion at ambient and cryogenic temperatures.However,Ni cannot effect-ively block the corrosion of Cl-,so corrosion often occurs in environment containing Cl-[4-5].The Cl-present in ink is the main reason for the surface corrosion of sample 3#,whereas organic ingredients have no influence on the surface.In a high-tem-perature and high-humidity environment,moisture preferentially adheres to the surface,which provides corrosion conditions for Cl-.With the dissolved oxygen in water,the coexistence of three factors,i.e.,O2-,H2O,and Cl-,results in a corrosive environment.First,Cl-penetrates the passivation film of the Fe-36%Ni surface,then preferentially combines with Fe2+and H2O to produce FeCl2·4H2O.The hydrate FeCl2·4H2O then decomposes into Fe(OH)2,Cl-,and H+.The released H+neutralizes OH-around the passivation film,thus causing local acidification and eventual pitting cor-rosion[6-8].Subsequently,Fe(OH)2is oxidized to a reddish-brown Fe(OH)3.This Fe(OH)3is quite unstable and decomposes into Fe2O3when heated.These chemical reaction processes are briefly desc-ribed in Equations (1)-(4).

Fe2++2Cl-+4H2O=FeCl2·4H2O

(1)

FeCl2·4H2O=Fe(OH)2+2Cl-+2H++2H2O

(2)

4Fe(OH)2+2O2-+2H2O=4Fe(OH)3

(3)

2Fe(OH)3=Fe2O3+3H2O

(4)

Table 3 Composition of corrosion pits %

Fig.3 shows the surface composition of the four elements of O,Cl,Fe,and Ni in the corrosion and nearby areas.It can be clearly seen that the corrosion area has a higher O content than the alloy matrix.The area where part of the corrosion fell off has a similar O content to that of the alloy matrix.At the same time,the Fe and Ni contents in the corrosion area are lower than those in the matrix.Cl was detected throughout the analytical field of the marked area,and the Cl content in the corrosion area was slightly higher than that in the matrix.So,it is clear that Cl-plays a key role in the overall corrosion process,first by penetrating the surface passivation film and binding to Fe2+,then forming Fe2O3when oxidized by oxygen,and eventually causing the pitting corrosion of Fe-36%Ni.

3.2 Effect of time on corrosion

Fig.4 shows the changes in the corrosion mor-phologies of sample 3#after 120,250,500,and 1 000 h,respectively.It can be seen that a large amount of pitting had occurred after sample 3#had been in the environmental testing chamber for 120 h.The corrosion pits are connected and occur as irregular filaments.With increased holding time,the corrosion worsens and gradually deepens due to the presence of Cl-.After 250 h,the corrosion area enlarges and expands to the periphery.After 1 000 h,the corrosion areas overlap and begin to form powder.Therefore,it is clear that in the presence of Cl-,the corrosion of the alloy surface becomes more and more serious with time.

4 Discussion

Based on the above test results,it can be seen that Cl-plays a key role in the corrosion process of the Fe-36%Ni strip.During the production of Fe-36%Ni material and its application in LNG shipbuilding,the strip encounters a variety of environmental con-ditions that can lead to surface corrosion.Depending on the steps taken during the overall process,the strip may not be able to be used and must instead be scrapped since it could cause serious accidents once corrosion has occurred.Careful measures must be taken to prevent corrosion on the strip.When purchasing markers that will come into contact with the material surface,it is essential that the ink in the marker does not contain corrosive ions like Cl-,S2-or any other.The presence or absence of these ions can be ascertained by checking the warranty of the marker or conducting a chemical test.During surface cleaning,it is also important that the reagents contain no corrosive ions.In addition,workers should avoid any direct hand contact with the strip surface because of the Cl-present in human perspiration.All items that come into contact with the strip surface must be subject to rigorous testing to prevent its interaction with any corrosive ions like Cl-.It is also necessary to ensure reasonable temperature and humidity conditions during storage to avoid surface dew that can lead to the formation of a corrosive environment.

5 Conclusions

(1) The existence of Cl-in ink is the main cause of corrosion on the surface of Fe-36%Ni alloy.High temperature and high humidity create a corrosive environment that promotes corrosion.

(2) After 120 h in a high-humidity and high-temperature environment,pitting corrosion occurred on the surface of Fe-36%Ni by the corrosive effect of Cl-.The observed corrosion areas were bright red and irregular in shape on the areas that had been marked,and mainly comprised Fe and O.With increased time,the corrosion area expanded to nearby areas and turned to powder,which eventually caused a decrease in the contents of Fe and Ni and increases in the contents of O and Cl in the marked areas.

(3) During the production of Fe-36%Ni material and its application in LNG shipbuilding,care must be taken in the selection of the marker or medium that will come into contact with the alloy surface to ensure that it contains no corrosive ions like Cl-.The alloy must also be stored at appropriate humidity and temperature to prevent corrosion and keep the alloy surface clean.