WO2018132967A1

WO2018132967A1 - Corrosion inhibiting coating

Info

Publication number: WO2018132967A1
Application number: PCT/CN2017/071522
Authority: WO
Inventors: Changjian LIN; Jinshan Pan; Per M. Claesson; Fan Zhang; Chengdong CHEN; Ruiqing HOU; Shigang DONG
Original assignee: Biopolymer Products Of Sweden Ab
Priority date: 2017-01-18
Filing date: 2017-01-18
Publication date: 2018-07-26

Abstract

A method for corrosion protection of metal objects comprises contacting the metal objects with a combination of at least one cerium oxide and at least one polymer comprising at least one mussel adhesive protein, and subsequently heating the coating to at least 60°C for at least 10 minutes. The material is environmentally friendly and does not show serious health risks as some compounds in the art. Only small amounts of coating material are required. The combination of mussel adhesive protein and small particles of cerium oxide gives the excellent corrosion protection in particular after heating. The combination of mussel adhesive protein, cerium oxide and phosphate ions gives a composition that is very suitable to cure with heating to give a further improved corrosion inhibiting coating which is better than that in the prior art.

Description

Corrosion inhibiting coating

Technical field

The present invention relates generally to an environmentally friendly coating for the prevention of corrosion of metals as well as a method for applying the coating.

Background

Corrosion is a metallic material degradation process occurring naturally， which can cause catastrophic accidents and huge economic losses. It can be controlled by suitable anti-corrosion strategies， and application of coatings is one of the most effective approaches. However， traditional one often involves chromate or organic phosphoric acid baths， which can cause environmental pollution and health risk. It is necessary to develop alternative environmental friendly pretreatment techniques.

It is known that cerium oxide treatment of a metal improves the corrosion resistance due to the formation of a protective oxide film which acts as an active protective layer on the metal surface.

Adhikari et al in Electrochimica Acta， Vol 53， issue 12， pp 4239-4247 studies anticorrosion properties of a coating comprising modified polyaniline dispersed in polyvinylacetate on carbon steel.

Zhitomirsky in Surface Engineering， Vol 20， issue 1， pp. 43-47 discloses electrodeposition of films comprising ceria and the cationic polymer polyethylenimine.

Corrosion inhibiting coatings according to the state of the art often use compounds which are known to cause environmental problems and/or health problems for users. Examples include chromium compounds.

Y. Gao et al in Transactions of the Institute of Metal Finishing vol 84， no3， 2006， pp 141-148 discloses corrosion protection of zinc electroplated steel. The corrosion inhibiting coating is a coating comprising either gelatin or albumin as well as dichromate. Also an alternative coating comprising gelatin and cerium trichloride is disclosed. It is concluded that the ability of cerium trichloride to stabilize protein formulations against putrefaction is questionable and that its adoption would require an associated stabilizer.

US 2004/0028820 discloses coating of aluminum using cerium ions in the presence of an oxidizing agent. The preferred cerium-based coatings comprise cerium oxide， hydrated cerium oxide， or forms of cerium hydroxide after coating. The coating bath optionally contains animal gelatin， glycerol， or other organic additive to improve coating uniformity and corrosion resistance. It is speculated that the gelatin functions to modify the nucleation and growth sites.

Mussel adhesive protein (MAP) is formed in a gland in the foot of byssus forming mussels， such as the common blue mussel (Mytilus edulis) . US 5,015,677 as well as US 4,585,585 disclose that MAP has very strong adhesive properties after oxidation and polymerization， e.g. by the activity of the enzyme tyrosinase， or after treatment with bifunctional reagents.

J.H. Waite et al in The Journal of Adhesion， vol. 81， 2005， pp 297-317 reviews adhesive proteins from mussels.

Lee et al in Science， vol 318， 2007， pp 426-430 discloses dopamine self-polymerization to form thin， surface-adherent polydopamine films onto a wide range of inorganic and organic materials， including noble metals， oxides， polymers， semiconductors， and ceramics.

WO 03/008376 discloses conjugation of DOPA moieties to various polymeric systems.

A. Statz et al in Biofouling， vol 22， no 6， 2006， pp 391-399 concerns marine antifouling and fouling-release performance of titanium surfaces coated with a polymer consisted of methoxy-terminated poly (ethylene glycol) conjugated to the adhesive amino acid DOPA and was chosen based on its successful resistance to protein and mammalian cell fouling. It is concluded that this polymer may be effective in marine antifouling and fouling-release applications.

CN 101658837 discloses preparation of an anticorrosive film for metal surfaces. The film comprises dopamine.

WO 03/080137 discloses a method for attaching two surfaces using a protein and periodate ions.

Mussel adhesive protein (MAP) derived from marine mussel Mytilus edulis， is a non-toxic and environmental friendly material， has shown its universal adhesive， film-forming and corrosion inhibiting properties for metals. It contains a high di-hydroxyphenylalanine (DOPA) content， which exhibits an excellent ability to form complexes with metal ions and metal oxides.

WO 2012/007199 discloses a corrosion inhibiting coating comprising at least one cerium oxide and at least one polymer. The polymer comprises at least one cathecholic component covalently bound thereto， and the at least one polymer displays a net positive charge at a pH of 7. The polymer can be for instance a mussel adhesive protein.

Although the corrosion protection in the prior art is working fine， there is still a need for a further improved corrosion protection.

Summary

It is an object of the present invention to alleviate at least some of the disadvantages of the prior art and to provide an improved coating for at least partially preventing corrosion of metals.

In a first aspect there is provided a coating for corrosion protection of metal objects， said coating comprising at least one cerium oxide and at least one polymer， wherein the at least one polymer comprises at least one mussel adhesive protein， wherein the coating has been subjected to heating to at least 60℃ during at least 10 minutes.

In a second aspect there is provided a method for corrosion protection of metal objects， said method comprising contacting a metal object with at least one cerium oxide and at least one polymer， wherein the at least one polymer comprises at least one mussel adhesive protein to obtain a coating， and subsequently heating the coating to at least 60℃ during at least 10 minutes.

Further embodiments of the aspects are described in the appended claims.

Advantages of the invention include that the material is environmental friendly and does not display the serious health risks as the compounds according to the state of the art. Further an excellent corrosion inhibition is obtained. Moreover， only small amounts of coating material are required.

The combination between the polymer and small particles comprising cerium oxide gives the excellent corrosion protection in particular after heating. The combination of mussel adhesive protein， cerium oxide and phosphate ions give a composition that is very suitable to cure with heat to give a further improved corrosion inhibiting coating which is better than in the prior art.

The coating displays a strong binding to the surface. The corrosion inhibiting properties are excellent for many metals and even for carbon steel.

Brief description of the drawings

The invention is described with reference to the following drawings in which：

Fig. 1 shows the morphology of the composite film after heating in secondary electron mode SEM images.

Fig. 2 shows EIS spectra of the nanocomposite film on carbon steel without heating (a) and with heating (b) ， after different time periods in neutral 0.1 M NaC1 solution. The graphs are from example 3.

Detailed description

Before the invention is disclosed and described in detail， it is to be understood that this invention is not limited to particular compounds， configurations， method steps， substrates， and materials disclosed herein as such compounds， configurations， method steps， substrates， and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

It must be noted that， as used in this specification and the appended claims， the singular forms ″a″ ， ″an″ and ″the″ include plural referents unless the context clearly dictates otherwise.

If nothing else is defined， any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains.

The term ″about″ as used in connection with a numerical value throughout the description and the claims denotes an interval of accuracy， familiar and acceptable to a person skilled in the art. Said interval is i 10 ％of the numerical value.

All percentages are calculated by weight unless otherwise clearly indicated.

As used throughout the claims and the description， the term ″metal object″ denotes an object comprising at least partially a metal surface. An object made of a metal and a non-metal where a part of the surface is a metal surface is thus encompassed within the term metal object. Further objects at least partially made of different metals as well as metal alloys are encompassed within the term.

As used throughout the claims and the description， the term ″coating″ denotes a covering that is applied at least partially to the surface of an object.

As used throughout the claims and the description， the term ″cerium oxide″ denotes a chemical compound or complex comprising the chemical element cerium (Ce) and the chemical element oxygen (O) . The term ″cerium oxide″ denotes oxides of cerium including Ce₂O₃ and CeO₂. The terms ceric oxide， ceria， cerium (III) oxide， cerium (IV) oxide and cerium dioxide are also encompassed by the term cerium oxide.

As used throughout the claims and the description， the term ″marine organism″ denotes water living organisms.

As used throughout the claims and the description， the term ″mussel″ denotes several families of the bivalvia molluscs including but not limited to the family mytilidae.

As used throughout the claims and the description， the term ″byssus forming mussels″ denotes bivalvia molluscs forming byssus.

As used throughout the claims and the description， the term ″carbon steel″ denotes alloys comprising more than 50 wt％iron and with a carbon content of less than 2 wt％. Steel is considered to be carbon steel when no minimum content is specified or required for chromium， cobalt， molybdenum， nickel， titanium， tungsten， vanadium and zirconium or any other element to be added to obtain a desired alloying effect.

The net charge of the polymer often varies with the pH depending on the nature of the polymer. For groups of the polymer may have a charge which varies with the pH. The pH of the polymer is preferably so that the net charge of the polymer (mussel adhesive protein) is positive at the application of the polymer. At pH 7 there may be both positive and negative charges on the polymer， but the net charge of a polymer is positive at pH 7， pi of MAP is about 10.3.

In one embodiment the at least one cerium oxide is CeO₂ (ceria) . In one embodiment the at least one cerium oxide is in the form of particles. In one embodiment the at least one cerium oxide is in the form of particles with a diameter of 1-1000 nm. It is an advantage to use particles with relatively small diameter. Examples of further size intervals for the particles include but are not limited to 4-80 nm， 4-40 nm， 5-50 nm， and 5-100 nm. Without wishing to be bound by any particular scientific theory the inventors believe that the particles of cerium oxide and the polymer form complexes， suitably together with phosphate and which complexes are extremely efficient regarding corrosion protection after the heat treatment.

In one embodiment the at least one polymer is at least one mussel adhesive protein extracted from a byssus-forming mussel.

In one embodiment the polymer is at least one mussel adhesive protein selected from the group consisting of MEFP-1， MEFP-2， MEFP-3， MEFP-4， MEFP-5， and MEFP-6. The abbreviations stand for Mytilus Edulis，

f

oot protein

1， 2， 3， 4， 5， and 6 respectively. In one embodiment the polypeptide is MEFP-1.

In one embodiment the coating comprises at least one layer comprising the at least one polymer， and wherein the coating further comprises at least one other layer comprising the at least one cerium oxide. In one embodiment the coating comprises a layer comprising both the at least one polymer and the at least one cerium oxide. In another embodiment the coating comprises two or more layers comprising the at least one polymer， and wherein the coating further comprises at least two or more layers comprising the at least one cerium oxide.

In one embodiment the coating is at least partially applied to a metal object. In one embodiment the coating is at least partially applied to an object comprising a metal. In one embodiment the metal is at least one metal selected from the group consisting of iron， zinc， aluminum， and copper. In another embodiment the metal is steel. In yet another embodiment the metal is carbon steel.

In one embodiment the coating is at least partially applied to an object comprising carbon steel.

In one embodiment the coating is at least partially applied to an object comprising a Fe-based alloy. An Fe-based alloy is considered to be an alloy with 50wt％of more of Fe.

In one embodiment the coating is at least partially applied to an object comprising a magnesium alloy. A magnesium alloy is an alloy with 50wt％or more of Mg.

In one embodiment the coating is at least partially applied to an object comprising an aluminum alloy. An aluminum alloy is an alloy with 50wt％or more of aluminum.

In one embodiment the coating comprises hydrogen phosphate. The inventors have noted that the combination of mussel adhesive protein， cerium oxide， and phosphate ions， in particular hydrogen phosphate gives a coating， which properties are very much improved upon heating so that its long term stability and efficiency is increased together with the protective properties.

The various embodiments of the first aspect are applicable also for the second aspect.

In one embodiment of the second aspect the metal object is contacted with at least one cerium oxide and at least one polymer by immersing.

In one embodiment of the second aspect the metal object is immersed between 10 minutes and 2 hours. In one embodiment of the second aspect the metal object is contacted with at least one cerium oxide and at least one polymer by spraying. In one embodiment of the second aspect the metal object is contacted with at least one cerium oxide and at least one polymer by roll coating. In one embodiment the contacting time for immersion is less than 1 min and up to several hours and even days， such as two days. The contacting time between the solution and the metal to be coated should not be too short. If the contacting time is too short the corrosion protection becomes less efficient. In one embodiment the contacting time is 30 minutes or more. In another embodiment the contacting time is 1 hour or more. In some experiments with certain conditions it has turned out that the corrosion protection does seldom increase significantly after a contacting time of more than 1 hour. The contacting time is the time during which the solution is in contact with the metal to be protected， for instance by immersion.

The heating should be performed toa temperature of at least 60℃. In one embodiment the heating should be performed to at least 75℃. In one embodiment the heating should be performed to at least 100℃. In one embodiment the heating should be performed to at least 150℃. In one embodiment the heating should be performed to at least 200℃. In one embodiment the heating should be performed to at least 250℃.

The heating should be performed during at least 10 minutes. In one embodiment the heating is performed during at least 20 minutes. In one embodiment the heating is performed during at least 40 minutes. In one embodiment the heating is performed during at least 1 hour. In one embodiment the heating is performed during at least 2 hours. In one embodiment the heating is performed during at least 3 minutes.

A shorter heating time can to some extent be compensated by a higher temperature. Thus if a short heating time is selected it is preferred to select a higher temperature in order to get the optimum corrosion protection. Vice versa if a low temperature is selected a longer heating time should be selected to get the best possible corrosion protection. A heating time of at least 10 minutes at 60℃ gives a good improvement compared to no heating at all.

In one embodiment of the second aspect at least one buffer solution is used and wherein the buffer solution comprises at least one selected from citric acid， phosphoric acid， citrate ions， and phosphate ions， hydrogen phosphate ions， and dihydrogen phosphate ions. In one embodiment the pH range of the solution comprising mussel adhesive protein and cerium oxide is in the range 2-9. This solution is used for contacting the metal to be protected. In an alternative embodiment the pH of the solution is in the range 4-9. In yet another embodiment the pH of the solution is in the range 6-7.

In one embodiment of the second aspect the concentration of the at least one polymer is 0.01-10 g/1 and the concentration of the at least one cerium oxide is 0.1-10 g/l， calculated for a solution/suspension used for contacting with the metal object. A skilled person realizes that the concentration increases as the solvent (water) evaporates after the contacting.

In one embodiment of the second aspect the concentration of phosphate ion (s) is 1-10 wt％calculated for a solution/suspension used for contacting with the metal object. The phosphate is at least one selected from the group consisting of phosphoric acid， dihydrogen phosphate， hydrogen phosphate and phosphate depending on the pH and concentrations as known to a person skilled in inorganic chemistry and chemical equilibriums. The percentage is calculated based on phosphate ions.

In one embodiment of the second aspect the method comprises the steps of： a) applying at least one layer comprising the at least one cerium oxide， and b) applying at least one other layer comprising the at least one polymer.

In one embodiment of the second aspect a layer comprising the at least one cerium oxide and a layer comprising the at least one polymer are applied sequentially several times.

In one embodiment of the second aspect the method comprises the step of applying a layer， said layer comprising both the at least one cerium oxide and the at least one polymer. In alternative embodiments several such layers are applied.

In one embodiment of the second aspect the application is performed using at least one method selected from the group consisting of spraying and dipping.

In one embodiment of the second aspect the polymer is oxidized. In one embodiment of the second aspect the polymer is cross-linked. In one embodiment the polypeptide is oxidized by addition of an oxidant. In one embodiment the polypeptide is oxidized using periodate ions. In one embodiment the polypeptide is oxidized by increasing the pH to 8 or above. The oxidation and cross-linking creates excellent adhesion and covalent bonds between the polymer chains and to oxides on the surface as well as to the particles comprising cerium oxide.

In one embodiment of the second aspect the metal object is ground prior to contacting the metal object with the at least one cerium oxide and the at least one polymer. In one embodiment the surface of the metal object is ground with a grinding paper successively from about 500 to 2400 grits. In one embodiment the grinding paper is a SiC grinding paper. In one embodiment the surface of the metal object is cleaned using ultrasound. In one embodiment the surface of the metal object is cleaned using a solvent. One example of a solvent includes but is not limited to ethanol.

Normally it takes some time for the coating to become an efficient corrosion protective coating. As evident from the examples this may typically take a couple of days for the coating to gradually become more and more efficient. This is normally not a problem since the corrosion protective coating usually is effective during a long time such as many years and the skilled person can adapt the manufacturing process with the delayed effectiveness in mind.

Without wishing to be bound by any particular theory the inventors believe that the oxidizing ability of the MAP and ceria， can form a protective oxide (e.g.， Fe₂O₃) on for instance a carbon steel surface beneath (or incorporated into) the coating， in particular together with phosphate ions and that the coating becomes more efficient with the heating.

Examples

All the solutions used in this method are prepared using analytical grade reagents and Milli-Q water. The operations without detailed mention in this method are commonly used. The MAP used in this method is supplied by Biopolymer Products AB (Gothenburg， Sweden) .

Example 1

(a) . Using ground with SiC grinding papers to grind carbon steel samples successively from 500 to 2400 grits， and then cleaning ultrasonically in ethanol

(b) . Preparing a mixture solution containing 0.05 g/L MAP (diluted with 1％citric acid solution) and 0.5 g/L CeO₂ nanoparticles for use.

(c) . Immersing the carbon steel samples into the mixture solution for 30 minutes to form a composite film.

(d) . Heating the preformed film samples in 200 ℃ for 10 minutes to obtain the final composite film.

With treatment by this method， the composite film provides a good and increasing corrosion protection for carbon steel.

Example 2

(b) . Preparing a mixture solution containing 0.05 g/L MAP (diluted with 1％citric acid solution) ， 0.5 g/L CeO₂ nanoparticles and 5 wt. ％Na₂HPO₄ for use.

Example 3

A composite film is obtained to provide good corrosion protection for carbon steel in the same manner as in Example 1 except the heating temperature and time are changed to 100 ℃for 1 hour. The experiment was repeated with identical conditions but without heating and the comparative results are shown in fig 2 and table 1.

Table 1. Fitting results for the samples with the nanocomposite film before and after the heating. EIS spectra were recorded in 0.1 M NaC1 solution.

Example 4

A composite film is obtained to provide good corrosion protection for carbon steel in the same manner as in Example 2 except the heating temperature and time are changed to 100 ℃for 1 hour.

Example 5

A composite film is obtained to provide good corrosion protection for carbon steel in the same manner as in Example 1 except the heating temperature and time are changed to 150 ℃for 30 minutes.

Example 6

A composite film is obtained to provide good corrosion protection for carbon steel in the same manner as in Example 2 except the heating temperature and time are changed to 150 ℃for 30 minutes.

Results and discussion

Fig. 1 shows the morphology of the composite film after heating in secondary electron mode SEM images. The morphology of the composite film exhibits the polishing scratches， defects and aggregates which are MAP/CeO₂/Na₂HPO₄ as revealed from the EDS analysis. The micrograph is from example 3.

Fig. 2 presents the EIS spectra of the carbon steel with the composite film after heating， after different time periods in the neutral 0.1 M NaC1 solution. It is clearly shown that， the polarization resistance was iow initially (2 hour) but increased gradually with time of exposure， and approached a high level after 3 days， thus it takes some time for the composite film to become protective. Therefore， the composite film prepared by this method can provide good and increasing corrosion protection for carbon steel. It can be seen that the heating makes the coating more compact and uniform which gives an enhanced corrosion protection.

Table 1 above displays the fitting results for the EIS spectra of the samples with the composite film after heating in 0.1 M NaC1 solution. The data are from example 3. The Rp value was 1.6 kΩ cm² in the beginning (2 h) ， and then increased to around 3.5 kΩ cm² after exposure to the neutral 0.1 M NaC1 solution for 2 or 3 days， indicating the composite film presented an increasing corrosion protection for carbon steel.

Claims

A coating for corrosion protection of metal objects， said coating comprising at least one cerium oxide and at least one polymer， wherein the at least one polymer comprises at least one mussel adhesive protein， wherein the coating has been subjected to heating to at least 60℃ during at least 10 minutes.
The coating according to claim 1， wherein the at least one cerium oxide is CeO₂.
The coating according to any one of claims 1-2， wherein the at least one cerium oxide is in the form of particles with a diameter of 1-1000 nm.
The coating according to any one of claims 1-3， wherein the at least one polymer is at least one mussel adhesive protein extracted from a byssus-forming mussel.
The coating according to any one of claims 1-4， wherein the polymer is at least one mussel adhesive protein selected from the group consisting of MEFP-1， MEFP-2， MEFP-3， MEFP-4， MEFP-5， and MEFP-6.
The coating according to any one of claims 1-5， wherein the coating comprises at least one layer comprising the at least one polymer， and wherein the coating further comprises at least one other layer comprising the at least one cerium oxide.
The coating according to any one of claims 1-6， wherein the coating comprises a layer comprising both the at least one polymer and the at least one cerium oxide.
The coating according to any one of claims 1-7， wherein the coating is at least partially applied to a metal object.
The coating according to any one of claims 1-8， wherein the coating is at least partially applied to an object comprising carbon steel.
The coating according to any one of claims 1-9， wherein the coating is at least partially applied to an object comprising a Fe-based alloy.
The coating according to any one of claims 1-10， wherein the coating is at least partially applied to an object comprising a magnesium alloy.
The coating according to any one of claims 1-11， wherein the coating is at least partially applied to an object comprising an aluminum alloy.
The coating according to any one of claims 1-12， wherein the coating comprises hydregen phosphate.
A method for corrosion protection of metal objects， said method comprising contacting a metal object with at least one cerium oxide and at least one polymer， wherein the at least one polymer comprises at least one mussel adhesive protein to obtain a coating， and subsequently heating the coating to at least 60℃ during at least 10 minutes.
The method according to claim 14， wherein the at least one cerium oxide is CeO₂.
The method according to any one of claims 14-15， wherein the at least one cerium oxide is in the form ef particles with a diameter of 1-1000 nm.
The method according to any one of claims 14-16， wherein the at least one polymer is at least one mussel adhesive protein extracted from a byssus-forming mussel.
The method according to any one of claims 14-17， wherein the polymer is at least one mussel adhesive protein selected from the group consisting of MEFP-1， MEFP-2， MEFP-3， MEFP-4， MEFP-5， and MEFP-6.
The method according to any one of claims 14-18， wherein the coating is applied as at least one layer comprising the at least one polymer， and at least one other layer comprising the at least one cerium oxide.
The method according to any one of claims 14-19， wherein the coating is applied as a layer comprising both the at least one polymer and the at least one cerium oxide.
The method according to any one of claims 14-20， wherein the coating is at least partially applied to a metal object.
The method according to any one of claims 14-21， wherein the coating is at least partially applied to an object comprising carbon steel.
The method according to any one of claims 14-22， wherein the coating is at least partially applied to an object comprising an Fe-based alloy.
The method according to any one of claims 14-23， wherein the coating is at least partially applied to an object comprising a magnesium alloy.
The method according to any one of claims 14-24， wherein the coating is at least partially applied to an object comprising an aluminum alloy.
The method according to any one of claims 14-25， wherein the coating comprises hydregen phosphate.
The method according to any one of claims 14-26， wherein the metal object is contacted with at least one cerium oxide and at least one polymer by immersing.
The method according to claim 27， wherein the metal object is immersed between 10 minutes and 2 hours.
The method according to any one of claims 14-28， wherein the metal object is contacted with at least one cerium oxide and at least one polymer by spraying.
The method according to any one of claims 14-29， wherein the metal object is contacted with at least one cerium oxide and at least one polymer by roll coating.
The method according to any one of claims 14-30， wherein at least one buffer solution is used and wherein the buffer solution comprises at least one selected from citric acid， phosphoric acid， citrate ions， and phosphate ions， hydrogen phosphate ions， and dihydrogen phosphate ions.
The method according to any one of claims 14-31， wherein the concentration of the at least one polymer is 0.01-10 g/1 and the concentration of the at least one cerium oxide is 0.1-10 g/l， calculated for a solution/suspension used for contacting with the metal object.
The method according to any one of claims 14-32， wherein the concentration of phosphate is 1-10 wt％ calculated for a solution/suspension used for contacting with the metal object.
The method according to any one of claims 14-33， wherein the method comprises the steps of： a) applying at least one layer comprising the at least one cerium oxide， and b) applying at least one other layer comprising the at least one polymer.
The method according to any one of claims 14-34， wherein a layer comprising the at least one cerium oxide， and a layer comprising the at least one polymer are applied sequentially several times.
The method according to any one of claims 14-33， wherein the method comprises the step of applying a layer， said layer comprising both the at least one cerium oxide and the at least one polymer.
The method according to any one of claims 14-36， wherein the application is performed using at least one method selected from the group consisting of spraying and dipping.
The method according to any one of claims 14-37， wherein the polymer is oxidized.
The method according to any one of claims 14-38， wherein the polymer is cross-linked.
The method according to any one of claims 14-39， wherein the metal object is ground prior to contacting the metal object with the at least one cerium oxide and the at least one polymer.