DAMAGE MECHANISMS (LOWT) QUIZ
HERE’S THE ANSWERS

Qu1. Atmospheric LOWT corrosion

A lot of LOWT on operating plant is caused by straightforward atmospheric corrosion. For low carbon steel which is externally coated, LOWT caused by atmospheric corrosion is likely to be;

a) Localised or general
b) Around welds
c) General
d) Localised

Ans (d). API 571(4.3.2.5). This is a ‘value-judgement’ answer rather than a direct quote. Section 4.3.25(b) says that if there is no coating, attack will be general but for coated it has a tendency to be localised, starting from areas of local coating breakdown.

Qu2. Elevated temperature LOWT corrosion.

For unlagged vessels or pipework operating above 175 degC, atmospheric corrosion;

a) Is unlikely
b) Increases as temperature increases
c) Decreases as temperature decreases
d) Stays at a constant rate

Ans (a) API 571 (4.2.3.2f).It is too hot for moisture condensation.

Qu3. Dissimilar weld corrosion.

Dissimilar welds can be susceptible to LOWT corrosion. The most vulnerable region in a dissimilar metal weld is;

a) Almost anywhere on or near the weld
b) Melted areas
c) The weld root
d) The area influenced by heat input but not melted

Ans (d). General knowledge – not directly quoted in (4.3.1). This is the definition for HAZ which becomes anodic/vulnerable due to its large-grain microstructure.

Qu4. Acidic LOWT corrosion

Acids play a large part in LOWT corrosion found in refineries. Which of these refinery-specific LOWT DMs is not the result of acidic corrosion?

a) Amine corrosion
b) Hydrogen Sulphide (H2S) corrosion
c) Ammonium bisulphide corrosion
d) Phenol corrosion

Ans (c) API 571 (5.1.1.2.1a). Ammonium bisulphide is Alkaline sour water.

Qu5. Acidic LOWT corrosion

Some chemicals found in refineries have serious LOWT corrosion effects on low plain carbon steels. Which of these LOWT acidic DM causes corrosion not by the chemical itself, but by gases dissolved in it?

a) Amine corrosion
b) Ammonium chloride corrosion
c) Napthenic acid corrosion
d) Hydrofluoric acid corrosion

Ans (a). This is a direct quote from API 571 (5.1.1.1a).

5-MINUTE COACHING

LOWT DAMAGE MECHANISMS

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