Corrosion: How does oxygen passivate metals?

Contents

What Is Passivation

Fundamentals of passivation

Iron vs chromium

Mechanism of oxide passivation: How stress develops on oxide deposit

Pilling-Bedworth ratio (PBR)

How and why does stress develop on an oxide scale

Oxidation of alloys

Advantages of chromium

Stresses in oxide growth

We often wonder "how and why metal passivation works." There are two obvious concerns: [1] How does an oxide layer form on the surface of a metal? [2] Why can't all metals be passivated by oxygen? I have written the post with reference to iron and chromium to explain why oxide does not protect iron while the oxide layer on chromium makes the metal impervious to oxygen and corrosion. The two key advantages of chromium compared to iron are [1] it is less electropositive than iron. It has less affinity for oxidation to form oxide. [2] The most important advantage of chromium is it has only three oxides while iron has seventeen known iron oxides and oxyhydroxides of different volumes. The creation of so many different types and different molecular size oxides of different volumes generate much more stress on the metal surface than chromium leading to flaking out of the metal. This post will try to answer the questions. I hope you will find this note useful.

What is passivation?

Passivation is a metal finishing process that prevents corrosion. It is the process of forming an exterior layer on the surface of a metal part or component. Because of this exterior layer, the underlying metal will not be directly exposed to the surrounding atmosphere. Rather, the underlying metal will be sealed, making it more corrosion-resistant.

Metals are easily oxidized by oxygen. While a nonporous / no crack / fully developed oxide film can protect the metal from corrosion, oxygen diffusion through an undeveloped oxide film can destroy it.

Metal corrodes as a result of oxidation. When a metal, particularly iron or iron alloys, is exposed to oxygen, a chemical reaction occurs. Both the air and the water contain oxygen. When metal is exposed to air or water, a chemical reaction occurs that alters its physical properties. The iron will essentially oxidize, resulting in corrosion. By shielding metal parts and components with an external layer, passivation can protect them from corrosion.

Fundamentals

Oxygen has a strong affinity for metals.

The question is ‘why’?

Oxygen

Oxygen is strongly electronegative with a strong desire/affinity for electrons

Oxygen is a very reactive element. It’s a small atom. Its atomic number is eight. It has eight protons and eight neutrons in the nucleus and there are eight electrons distributed over two shells outside the nucleus.??It is short of electrons to achieve a stable atomic structure and therefore it always looks for electrons to achieve a stable atomic structure and this is supported by its tiny size [see periodic table ].?

The oxygen atom, because of its small size with its outer shell electrons being very close to positive protons in the nucleus, has the strong attraction of protons to acquire electrons in the outer shell. This makes oxygen a very strong electronegative element next to fluorine and also makes it a very reactive element. It oxidizes almost every metal barring noble metals. It is a very strong oxidizing agent. By definition, an oxidizing agent is a substance that makes others lose electrons and get itself reduced by acquiring those electrons.

Metals

Metals are electropositive with aversion/dislike for electrons

Opposite to oxygen, metals are much bigger in size with several layers of electrons outside the nucleus each repelling the other because of the same charge. ?This makes outer electrons of metals located far away from positive protons in the nucleus have practically no attraction for protons. Metals achieve their stable atomic structure by losing electrons, just opposite to oxygen.

Iron and Chromium atom?

Key properties

Iron- Atomic number 26, Electronegativity 1.8, hardness – 4 .0 in mho scale

Chromium- Atomic number 24, Electronegativity 1.6, hardness, 8.5 in mho scale

What does it mean?

Iron has a greater tendency to lose electrons. Iron has a greater attraction for oxygen to form oxide. Let us take iron as an example. It is a big atom [see periodic table] Its atomic number is twenty-six. It has twenty-six protons in the nucleus and thirty neutrons. Positive protons in the nucleus are shielded by four layers of electrons making the outer electrons practically have no attraction for positive protons in the nucleus. Therefore, iron like other metals prefer to lose outer electrons to achieve a stable atomic structure. This is what makes metals electropositive.

Despite oxygen’s strong affinity for metals to form oxides, there are only a few metal oxides that form a stable protective oxide layer on metals and protect them from corrosion. ?

Iron and chromium oxide molecules

LHS image: Iron oxide molecule????RHS image: Chromium oxide molecule [ not on the scale]

Iron oxide

Chromium oxide

Iron vs Chromium

Summary

The chromium, as the images are showing has an atomic number of 24 compared to the atomic number of iron is 26. Therefore, iron has more affinity to attract oxygen to form oxides. Chromium is almost twice as harder as iron. Chromium oxide, CrO3 is more compact and strongly bonded to oxygen. CrO3 is more impermeable to oxygen than Fe2O3.?

Mechanism of oxide passivation

Several factors can obstruct oxide growth on metal surfaces. The oxide-growth process occurs through either metal ion diffusion through the oxide or oxygen ion diffusion in the opposite direction. Several factors cause stress during the diffusion process. The higher the stress, the more fragile and porous the oxide deposit.

What does it mean?

How and why does stress develop on an oxide scale?

The generation of stress in an oxide scale can lead to scale cracking and spallation, [fragmenting the scale] affecting the protective oxide scale's maintenance. There are two kinds of stresses in oxide scales: growth stress, which occurs during the oxidation process, and thermal stress, which occurs as a result of the differential thermal expansion of the oxide scale and metal substrate.

Although the precise cause of the growth stress is complex and unknown, it is believed that the stress is strongly affected by the volumes of metal and oxide, the crystal structures of oxide and metal, and the growth mechanism of the oxide. When an oxide forms at the metal/oxide interface, the volume change due to the formation of the oxide can be expressed with the Pilling-Bedworth ratio (PBR)

Pilling-Bedworth ratio (PBR)

PBR = Volume of metal oxide / Volume of metal in the oxide

Since 1923, PBR has been used to predict the sign and magnitude of growth stress. It is generally accepted that when PBR >1, compressive stress develops in the oxide scale, whereas when PBR 1, tensile stress develops. The greater the difference in PBR from one, the greater the growth stress. However, it is also acknowledged that there is no direct relationship between PBR and the stress level on the scale. This means that the mechanisms of stress generation and release are complex, and the effects of PBR are not simple or well understood. Nonetheless, PBR has been frequently cited for explaining stress generation during oxidation, and it served as the foundation for the development of some recent models.

The Pilling-Bedworth ratio was established for use in the oxidation of metals. However, these are alloys that are widely used as high-temperature materials in practice.

Oxidation of alloys

When an alloy is exposed to an oxidizing atmosphere at high temperatures, one or more of its elements will oxidize. A noble parent metal alloying with a base element and a base parent element alloying with a base element are the two types of alloys. Only B will oxidize in a binary alloy A-B composed of a noble element A and a base element B. If there are two base elements A and B, only B will oxidize if its concentration exceeds that of A.

Summary:

If the PBR ratio is less than one, the oxide layer will be unprotective because the film that forms on the metal surface is porous and/or cracked.

Conversely, the metals with a ratio higher than 1 tend to be protective because they form an effective barrier that prevents the gas from further oxidizing the metal

General guidelines for PBR

RPB < 1: the oxide coating layer is too thin, likely broken, and provides no protective effect (for example magnesium)

RPB > 2: the oxide coating chips off and provides no protective effect (for example iron)

1 < RPB < 2: the oxide coating is passivating and provides a protective effect against further surface oxidation (for example aluminum, titanium, and chromium-containing steels).

Typical P-B ratio

Provides strong protective oxide film

Aluminium = 1.28

Chromium = 2.07

Iron (iii) oxide = 2.14

Advantages of chromium

Chromium is a hard metal with a hardness twice of iron. Despite its high PBR which is due to essentially spreading oxide deposit film on the surface of the metal because it forms a very hard oxide deposit on the alloy surface. Chromium doesn't oxidize nearly as easily as steel. Chromium is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a structure only a few atoms thick and very dense, preventing the diffusion of oxygen into the underlying material. The other advantages of chromium compared to iron are [1] it is less electropositive than iron. It has less affinity for oxidation to form oxide. The most important advantage of chromium is [2] it has only three oxides while iron has seventeen known iron oxides and oxyhydroxides of different volumes. The creation of so many different types and different molecular size oxides of different volumes generate much more stress on the metal surface than chromium leading to flaking out of the metal.

Key stresses that weaken oxide growth are as follows:

The geometry of Surfaces—The geometry of surfaces causes stress because the diffusion process can be modified, based on whether the surface is flat, concave, or convex. Thus, the geometry of a surface with the type of diffusing ion and its molar volume will generate stresses that can be tensile or compressive.

Thermal Stresses—These stresses are important as frequent heating and cooling happen in many industrial components. Thus, when a metal component is cooled from a rather high temperature, it can generate stresses that are generally compressive in nature.

When the oxide is unable to hold the stress, it needs to release these accumulated stresses. When the stresses are released, the protective oxide can form a microcrack or eventually lead to oxide failing.

Credit: Google?