Forms of Metallic Corrosion
29
3.1.2 Passive Film on Nickel
The passive film on nickel can be formed quite readily, in contrast to the
formation of the passive film on iron. Differences in the nature of the oxide
film on iron and nickel are responsible for this phenomenon. The thickness
of the oxide film on nickel is between 0.9 and 1.2 mm, whereas the iron oxide
film is between 1 and 4 mm. There are two theories as to what the passive
film on nickel is. It is either entirely NiO with a small amount of nonstoichiometry, giving rise to Ni 3+ cation vacancies, or it consists of an inner layer
of NiO and an outer layer of anhydrous Ni(OH) 2 . The passive oxide film on
nickel, once formed, cannot be easily removed by either cathodic treatment
or chemical dissolution.
The passive film on nickel will not protect the nickel from corrosive attack
in oxidizing atmospheres, such as nitric acid. When alloyed with chromium,
a much improved, stable film results, producing a greater corrosion resistance to a variety of oxidizing media. However, these alloys are subject to
attack in environments containing chlorides or other halides, especially if
oxidizing agents are present. Corrosion will be in the form of pitting. The
addition of molybdenum or tungsten will improve the corrosion resistance.
3.1.3 Passive Film on austenitic Stainless Steel
The passive film on austenitic stainless steel is duplex in nature, consisting
of an inner barrier oxide film and an outer deposit of hydroxide or salt film.
Passivation takes place by the rapid formation of surface-absorbed hydrated
complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that
rapidly deprotonates to form an insoluble surface oxide film. The three most
commonly used austenite stabilizers — nickel, manganese, and nitrogen —
all contribute to the passivity. Chromium, a major alloying ingredient, is in
itself corrosion resistant and is found in greater abundance in the passive
film than iron, which is the major element in the alloy.
3.1.4 Passive Film on Copper
When exposed to the atmosphere for long periods of time, copper will form
a coloration on the surface known as patina. In reality, the coloration is a corrosion product that acts as a protective film against further corrosion. When
first formed, the patina exhibits a dark color that gradually turns green. The
length of time required to form the patina depends upon the atmosphere
because the coloration is given by copper hydroxide compounds. In a marine
atmosphere, the compound is a mixture of copper/hydroxide/chloride; in
industrial atmospheres, it is copper/hydroxide/sulfate. These compounds
will form in approximately 7 years. When exposed in a clean rural atmosphere, tens or hundreds of years may be required to form the patina.
29
3.1.2 Passive Film on Nickel
The passive film on nickel can be formed quite readily, in contrast to the
formation of the passive film on iron. Differences in the nature of the oxide
film on iron and nickel are responsible for this phenomenon. The thickness
of the oxide film on nickel is between 0.9 and 1.2 mm, whereas the iron oxide
film is between 1 and 4 mm. There are two theories as to what the passive
film on nickel is. It is either entirely NiO with a small amount of nonstoichiometry, giving rise to Ni 3+ cation vacancies, or it consists of an inner layer
of NiO and an outer layer of anhydrous Ni(OH) 2 . The passive oxide film on
nickel, once formed, cannot be easily removed by either cathodic treatment
or chemical dissolution.
The passive film on nickel will not protect the nickel from corrosive attack
in oxidizing atmospheres, such as nitric acid. When alloyed with chromium,
a much improved, stable film results, producing a greater corrosion resistance to a variety of oxidizing media. However, these alloys are subject to
attack in environments containing chlorides or other halides, especially if
oxidizing agents are present. Corrosion will be in the form of pitting. The
addition of molybdenum or tungsten will improve the corrosion resistance.
3.1.3 Passive Film on austenitic Stainless Steel
The passive film on austenitic stainless steel is duplex in nature, consisting
of an inner barrier oxide film and an outer deposit of hydroxide or salt film.
Passivation takes place by the rapid formation of surface-absorbed hydrated
complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that
rapidly deprotonates to form an insoluble surface oxide film. The three most
commonly used austenite stabilizers — nickel, manganese, and nitrogen —
all contribute to the passivity. Chromium, a major alloying ingredient, is in
itself corrosion resistant and is found in greater abundance in the passive
film than iron, which is the major element in the alloy.
3.1.4 Passive Film on Copper
When exposed to the atmosphere for long periods of time, copper will form
a coloration on the surface known as patina. In reality, the coloration is a corrosion product that acts as a protective film against further corrosion. When
first formed, the patina exhibits a dark color that gradually turns green. The
length of time required to form the patina depends upon the atmosphere
because the coloration is given by copper hydroxide compounds. In a marine
atmosphere, the compound is a mixture of copper/hydroxide/chloride; in
industrial atmospheres, it is copper/hydroxide/sulfate. These compounds
will form in approximately 7 years. When exposed in a clean rural atmosphere, tens or hundreds of years may be required to form the patina.