API 571 (Damage Mechanisms Affecting Fixed Equipment)

Question 0

API 571

At what temperature does chloride stress corrosion cracking become a concern (Corrosion)?

Answer 0

API 571 (4) General Damage Mechanisms – All Industries
(4.5) Environment – Assisted Cracking
(4.5.1) Chloride Stress Corrosion Cracking (Cl-SCC)
(4.5.1.3) Critical Factors
>140°F
(4.5.1.3.g) Cracking usually occurs at metal temperatures above about 140°F (60°C), although exceptions can be found at lower temperatures.

Question 1

API 571

What is the typical dew point of hydrochloric acid (Corrosion)?

Answer 1

API 571 (4) General Damage Mechanisms – All Industries
(4.3) Uniform or Localized Loss of Thickness
(4.3.7) Flue-Gas Dew-Point Corrosion
(4.3.7.3) Critical Factors
130°F
(4.3.7.3.d) Similarly, the dewpoint of hydrochloric acid depends on the concentration of hydrogen chloride. It is typically about 130°F (54°C).

Question 2

API 571

Temperature swings that can cause thermal fatigue (Corrosion)?

Answer 2

API 571 (4) General Damage Mechanisms – All Industries
(4.2) Mechanical and Metallurgical Failure Mechanisms
(4.2.9) Thermal Fatigue
(4.2.9.3) Critical Factors
(4.2.9.3.c)
+/- 200°F
Startup and shutdown of equipment increase the susceptibility to thermal fatigue. There is no set limit on temperature swings; however, as a practical rule, cracking may be suspected if the temperature swings exceeds about 200°F (93°C).

Question 3

API 571

What is the most aggressive CUI temperature range (Corrosion)?

Answer 3

API 571 (4) General Damage Mechanisms – All Industries
(4.3) Uniform or Localized Loss of Thickness
(4.3.3) Corrosion Under Insulation (CUI)
(4.3.3.3) Critical Factors
(4.3.3.3.b)
212°F and 350°F
Corrosion rates increase with increasing metal temperature up to the point where the water evaporates quickly. For insulated components, corrosion becomes more severe at metal temperatures between the boiling point 212°F (100°C) and 350°F (121°C), where water is less likely to vaporize and insulation stays wet longer.

Question 4

API 571

What is the typical dew point of sulfuric acid (Corrosion)?

Answer 4

API 571 (4) General Damage Mechanisms – All Industries
(4.3) Uniform or Localized Loss of Thickness
(4.3.7) Flue-Gas Dew-Point Corrosion
(4.3.7.3) Critical Factors
(4.3.7.3.c)
280°F
The dewpoint of sulfuric acid depends on the concentration of sulfur trioxide in the flue gas, but is typically about 280°F (138°C).

Question 5

API 571

What is the starting temperature for Sulfidation in Iron-Based alloys in API 571 (Corrosion)?

Answer 5

API 571 (4) General Damage Mechanisms – All Industries
(4.4) High Temperature Corrosion [>400°F (204°C)]
(4.4.2) Sulfidation
(4.4.2.3) Critical Factors
(4.4.2.3.c)
500°F
Sulfidation of iron-based alloys usually begins at metal temperatures above 500°F (260°C). The typical effects of increasing temperature, chromium content and sulfur content on metal loss are shown in Figure 4-114 and Figure115.

Question 6

API 571

What is the most common material with an endurance limit (fatigue cracking cannot occur below this stress level) (Materials)?

Answer 6

API 571 (4) General Damage Mechanisms – All Industries
(4.2) Mechanical and Metallurgical Failure Mechanisms
(4.2.16) Mechanical Fatigue
(4.2.16.3) Critical Factors
(4.2.16.3.b) Metallurgical Issues and Microstructure
(4.2.16.3.b.1)
Carbon Steel
For some materials such as titanium, carbon steel and low alloy steel, the number of cycles to fatigue fracture decreases with stress amplitude until an endurance limit reached. Below this stress endurance limit, fatigue cracking will not occur, regardless of the number of cycles.

Question 7

API 571

What is a common material without an endurance limit (Materials)?

Answer 7

API 571 (4) General Damage Mechanisms – All Industries
(4.2) Mechanical and Metallurgical Failure Mechanisms
(4.2.16) Mechanical Fatigue
(4.2.16.3) Critical Factors
(4.2.16.3.b) Metallurgical Issues and Microstructure
(4.2.16.3.b.2)
Stainless Steel
For alloys with endurance limits, there is a correlation between Ultimate Tensile Strength (UTS) and the minimum stress amplitude necessary to initiate fatigue cracking. The ratio of endurance limit over UTS is typically between 0.4 and 0.5. Materials like austenitic stainless steels and aluminum that do not have an endurance limit will have a fatigue limit defined by the number of cycles at a given stress amplitude.

Question 8

API 571

What is the Ration of Endurance Limit Stress to Ultimate Tensile Stress for Carbon Steel (Misc. Numbers)?

Answer 8

API 571 (4) General Damage Mechanisms
(4.2) Mechanical and Metallurgical Failure Mechanisms
(4.2.16) Mechanical Fatigue
(4.2.16.3) Critical Factors
(4.2.16.3.b) Metallurgical Issues and Microstructure
(4.2.16.3.b.ii) 0.4-0.5
For alloys with endurance limits, there is a correlation between Ultimate Tensile Strength (UTS) and the minimum stress amplitude necessary to initiate fatigue cracking. The ratio of endurance limit over UTS is typically between 0.4 and 0.5. Materials like austenitic stainless steels and aluminum that do not have an endurance limit will have a fatigue limit defined by the number of cycles at a given stress amplitude.