Oxidation is a thermally activated process, meaning it becomes significant at high temperatures where the kinetics of the reaction are

favorable.
At room temperature, oxidation of most engineering alloys (e.g., carbon steel) is negligible, forming only thin, protective oxide layers. However, at
elevated temperatures, the reaction accelerates, leading to the formation of a thicker, often non-protective oxide scale.
252 API RECOMMENDED PRACTICE 571
Mechanism of Damage
Oxygen Reaction: Oxygen molecules from the environment (typically air) adsorb onto the metal surface. At high
temperatures, these molecules dissociate into atomic oxygen, which reacts with metal atoms to form metal oxides. For
3.48 Oxidation carbon steel, this results in iron oxides, while alloying elements in other materials (e.g., chromium in stainless steel)
may form their own oxides.
Formation of Oxide Scale: The oxide scale is a solid layer that grows on the metal surface. This scale is often
3.48.1 Description of Damage porous, brittle, or loosely adherent, depending on the material and conditions. The growth of the scale consumes metal
atoms from the substrate, effectively reducing the metal wall thickness.
Oxygen, most often present as a component of air (approximately 21 %), reacts with carbon steel and other alloys
at high temperature, converting the metal to oxide scale and thereby reducing the metal wall thickness.

3.48.2 Affected Materials

a) All iron-based materials including carbon steel and low-alloy steels, both cast and wrought, are affected.

b) All 300 series SS, 400 series SS, and nickel-based alloys also oxidize to varying degrees, depending on
composition and temperature.

3.48.3 Critical Factors

a) The primary factors affecting high-temperature oxidation are metal temperature and alloy composition.

b) Oxidation of carbon steel begins to become significant above about 1000 °F (540 °C). Rates of metal loss
increase with increasing temperature.

c) In general, the resistance of carbon steel and other alloys is determined by the chromium content of the
material. Increasing chromium levels produce a more protective oxide scale. The 300 series SS are resistant
to scaling up to about 1500 °F (815 °C). See Table 3-48-1a and Table 3-48-1b and Figure 3-48-1.

d) The presence of water vapor can significantly accelerate oxidation rates of some steels including 9Cr-1Mo.
(Reference 4)

3.48.4 Affected Units or Equipment

Significant oxidation occurs in fired heaters, boilers, and other combustion equipment, as well as piping and
equipment that operate in high-temperature, oxygen-containing environments where metal temperatures exceed
about 1000 °F (540 °C).

3.48.5 Appearance or Morphology of Damage

a) Carbon steel, low-alloy steels, and 12Cr stainless steels suffer general thinning due to oxidation. Usually, the
component will be covered on the outside surface with an oxide scale, depending on the temperature and
exposure time. (Figure 3-48-2 to Figure 3-48-4).

b) 300 series SS and nickel alloys generally have a very thin dark scale unless exposed to extremely high
temperatures where metal loss rates are excessive.

3.48.6 Prevention/Mitigation

a) Resistance to oxidation is best achieved by upgrading to a more resistant alloy.

b) Chromium is the primary alloying element that affects resistance to oxidation. Other alloying elements,
including silicon and aluminum, are effective, but their concentrations are limited due to adverse effects on
mechanical properties. They are often used in special alloys for applications such as heater supports, burner
tips, and components for combustion equipment.

3.48.7 Inspection and Monitoring

a) Process conditions should be monitored to establish trends of high-temperature equipment where oxidation
can occur.

b) Temperatures can be monitored with tube-skin thermocouples and/or infrared thermography.

 DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 253

c) RT can be used to measure remaining thickness when oxidation occurs on the external surface. Alternatively,
the oxide could be removed, e.g. using a flapper wheel, to allow UT measurement of the remaining wall
thickness.

d) UT can be used to measure remaining thickness when oxidation occurs on the internal surface.

e) EMAT has been used to measure general external wall loss on heater tubes.
EMAT: Electro Magnetic Acoustic Transducer
3.48.8 Related Mechanisms

Other high-temperature gas corrosion mechanisms are sulfidation (3.61), high-temperature H2/H2S corrosion
(3.35), carburization (3.13), and metal dusting (3.44). Oxidation damage referred to in this section is due to
surface scaling. Some damage mechanisms result in internal oxidation, which is outside the scope of this
document.

3.48.9 References

1. API Recommended Practice 581, Risk-Based Inspection Technology, American Petroleum Institute,
Washington, DC, Second Edition, 2008.

2. J. Gutzeit, R.D. Merrick, and L.R. Scharfstein, “Corrosion in Petroleum Refining and Petrochemical Operations,”
Metals Handbook, Volume 13, ASM International, Materials Park, OH, 1987, pp. 1262–1287.

3. Corrosion Basics—An Introduction, NACE International, Houston, TX, 1984, pp. 276–288.

4. F. Dettenwanger et al., “The Influence of Si, W and Water Vapor on the Oxidation Behavior of 9Cr Steels,”
Paper No. 01151, Corrosion/2001, NACE International, Houston, TX.

External Oxidation (Surface Scaling): Occurs at the metal surface, forming a visible
oxide layer that consumes metal and reduces thickness.
Internal Oxidation: Involves oxygen diffusion into the metal, forming oxide precipitates
within the microstructure. This can embrittle the material but does not directly reduce
wall thickness.

Internal oxidation is more common in alloys with elements like aluminum or silicon
that form stable oxides internally under specific conditions (e.g., low oxygen partial
pressure or high-temperature cycling).
254 API RECOMMENDED PRACTICE 571

Table 3-48-1a—Estimated Oxidation Rates (mpy) (Reference 1)

Maximum Metal Temperature (°F)

Material
925 975 1025 1075 1125 1175 1225 1275 1325 1375 1425 1475

CS 2 4 6 9 14 22 33 48 — — — —

1¼Cr 2 3 4 7 12 18 30 46 — — — —

2¼Cr 1 1 2 4 9 14 24 41 — — — —

5Cr 1 1 1 2 4 6 15 35 65 — — —

7Cr 1 1 1 1 1 2 3 6 17 37 60 —

9Cr 1 1 1 1 1 1 1 2 5 11 23 40

12Cr 1 1 1 1 1 1 1 1 3 8 15 30

304 SS 1 1 1 1 1 1 1 1 1 2 3 4

309 SS 1 1 1 1 1 1 1 1 1 1 2 3

310 SS/HK 1 1 1 1 1 1 1 1 1 1 1 2

800 H/HP 1 1 1 1 1 1 1 1 1 1 1 2

Maximum Metal Temperature (°F)

Material
1525 1575 1625 1675 1725 1775 1825 1875 1925 1975 2025 2075

CS — — — — — — — — — — — —

1¼Cr — — — — — — — — — — — —

2¼Cr — — — — — — — — — — — —

5Cr — — — — — — — — — — — —

7Cr — — — — — — — — — — — —

9Cr 60 — — — — — — — — — — —

12Cr 50 — — — — — — — — — — —

304 SS 6 9 13 18 25 35 48 — — — — —

309 SS 4 6 8 10 13 16 20 30 40 50 — —

310 SS/HK 3 4 5 7 8 10 13 15 19 23 27 31

800 H/HP 3 4 6 8 10 13 17 21 27 33 41 50
 DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 255

Table 3-48-1b—Estimated Oxidation Rates (mm/yr) (Reference 1, converted)

Maximum Metal Temperature (°C)

Material
495 525 550 580 605 635 665 690 720 745 775 800

CS 0.05 0.1 0.15 0.23 0.36 0.56 0.84 1.22 — — — —

1¼Cr 0.05 0.08 0.1 0.18 0.3 0.46 0.76 1.17 — — — —

2¼Cr 0.03 0.03 0.05 0.1 0.23 0.36 0.61 1.04 — — — —

5Cr 0.03 0.03 0.03 0.05 0.1 0.15 0.38 0.89 1.65 — — —

7Cr 0.03 0.03 0.03 0.03 0.03 0.05 0.08 0.15 0.43 0.94 1.52 —

9Cr 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05 0.13 0.28 0.58 1.02

12Cr 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.08 0.2 0.38 0.76

304 SS 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05 0.08 0.1

309 SS 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05 0.08

310 SS/HK 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05

800 H/HP 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.05

Maximum Metal Temperature (°C)

Material
830 855 885 915 940 970 995 1025 1050 1080 1105 1135

CS — — — — — — — — — — — —

1¼Cr — — — — — — — — — — — —

2¼Cr — — — — — — — — — — — —

5Cr — — — — — — — — — — — —

7Cr — — — — — — — — — — — —

9Cr 1.52 — — — — — — — — — — —

12Cr 1.27 — — — — — — — — — — —

304 SS 0.15 0.23 0.33 0.46 0.64 0.89 1.22 — — — — —

309 SS 0.10 0.15 0.20 0.25 0.33 0.41 0.51 0.76 1.02 1.27 — —

310 SS/HK 0.08 0.10 0.13 0.18 0.20 0.25 0.33 0.38 0.48 0.58 0.69 0.79

800 H/HP 0.08 0.10 0.15 0.20 0.25 0.33 0.43 0.53 0.69 0.84 1.04 1.27
256 API RECOMMENDED PRACTICE 571

Figure 3-48-1—Estimated oxidation rates based on the data in Table 3-46-1a.

Figure 3-48-2—Oxidation of a carbon steel nut on a stainless steel stud at 1300 °F (705 °C).
HIGH-TEMPERATURE OXIDATION

Description Appearance
A high-temperature scaling/ General LOWT under oxide
corrosion mechanism in plain scaled surface
carbon steels. Most common in
boilers and fired heaters

ir.
v*

</
r/

■-L

Inspection: UT to measure LOWT.

Macrograph to measure scale thickness on
boiler tube internal surfaces.
Critical factors: Will affect all iron-based materials.
Affects LCS at metal temperatures > 538°C
(1000°F) and 316SS > 816°C (1500°F).
Requires the presence of oxygen to occur.
FFP/Severity: Scaling on tube external surfaces can reduce
wall thickness to the point of leaking or
catastrophic failure.
Difficult to assess accurately to API 579 owing
to local scale/thinning concentrations.
References: API 571 (4.4.1)
 FIG D7

High-Temperature Oxidation
- Key Points -
Boiler
Remember: this is the tubes
high-temperature scaling Burner
type of oxidation

1000 F Low-carbon steel

LCS boiler components
"i

1500 F
:«»(»ss
Carbon steel
degrades to oxide
scale at high
temperature

Resistance is improved by increased chromium content ^

High resistance 1800°F

300 SS *

9 Cr Temperature
7 Cr
2KCr
IX Cr
LCS
Low resistance t
1000°F
 DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 257

Figure 3-48-3—Oxidation of a carbon steel grid from a sulfur reactor.

Figure 3-48-4—Oxidation of the OD of a carbon steel furnace transfer line.