API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
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API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

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2.B-10 API RECOMMENDED PRACTICE 5812.B.2.6FiguresStartNoIs thematerial C.S. or300 SeriesS.S.?YesYesDo you knowthe pH?No• Max. processtemperature• MaterialDetermine corrosionrate for Carbon Steeland 300 Series SSusing Tables 2.B.2.3and 2.B.2.4.Determine pHusing Table2.B.2.2Cl ConcentrationpHEstimatedCorrosion RateDo you knowthe Cl concentration ofthe Acidic Water?NoDetermine Chlorideconcentration fromTable 2.B.2.2.YesIs the material Alloy400 or Alloy B-2?NoYes• Material• Cl concentration• Temperature• Oxygen/Oxidant Present?DetermineCorrosion Rateusing Table2.B.2.6.DetermineCorrosion Rateusing Table2.B.2.5.• Material• Cl concentration• TemperatureEstimatedCorrosion RateEstimatedCorrosion RateFigure 2.B.2.1—HCl Corrosion—Determination of Corrosion Rate

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-112.B.3High Temperature Sulfidic and Naphthenic Acid Corrosion2.B.3.1 Description of DamageHigh temperature sulfidic corrosion is a form of normally uniform corrosion that can occur at temperaturestypically above about 204 °C (400 °F). This form of corrosion sometimes occurs along with naphthenic acidcorrosion depending on the oil being processed. Naphthenic acid corrosion, when it occurs, is normallylocalized. Sulfur species occur naturally in most crude oils but their concentrations vary from crude-to-crude.These naturally occurring compounds may be corrosive themselves as well as when they are converted tohydrogen sulfide through thermal decomposition. Catalytic conversion of sulfur compounds to 2S occurs in thepresence of hydrogen and a catalyst bed in hydroprocessing units. Corrosion in vapor streams containing bothH 2 S and hydrogen is covered in Section 2.B.4. As with sulfur compounds, naphthenic acids occur naturally insome crude oils. During distillation, these acids tend to concentrate in higher boiling point fractions such asheavy atmospheric gas oil, atmospheric resid, and vacuum gas oils. The acids may also be present in vacuumresid, but often many of the more corrosive ones will have distilled into the vacuum side streams. Lower boilingpoint streams are usually low in naphthenic acids. Corrosion may appear either as pitting, more common atlower acid levels, or grooving and gouging at higher acid levels and, particularly, at higher velocities.Naphthenic acids may modify or destabilize protective films (sulfides or oxides) on the material and thus allow ahigh sulfidation corrosion rate to continue or it may itself directly attack the base material.The corrosion rate in high temperature sulfidic environments is a function of the material, temperature, andthe concentration of the sulfur compound(s) present. The presence of naphthenic acid in sufficient amounts,however, can dramatically decrease a material’s corrosion resistance where it might otherwise have suitablecorrosion resistance. The following summarize the key variables in corrosion.a) In high temperature sulfidic environments, materials such as carbon and low alloy steels form sulfidecorrosion products. The extent to which these are protective depends on the environmental factorsmentioned. At high enough temperatures and/or sulfur levels, the corrosion products may become lessprotective so corrosion can occur at an accelerated rate.b) Moderate additions of chromium to carbon steel increase the material’s corrosion resistance. Alloyscontaining 5Cr-0.5Mo, 7Cr-1Mo, and 9Cr-1Mo are often sufficient to provide acceptable materialperformance in these environments. Lower alloys such as 1.25Cr-0.5Mo and 2.25Cr-1Mo generally donot offer sufficient benefits over carbon steel to justify their use. Stainless steels such as 12Cr (Type405, 410, 410S) and Type 304 may be required at higher sulfur levels and temperatures.c) Sulfidation corrosion is related to the amount of sulfur present in the stream and is usually reportedsimply as wt% sulfur. Corrosion generally increases with increasing sulfur content.d) High temperature sulfidic corrosion occurs at temperatures greater than about 204 °C (400 °F).Naphthenic acid corrosion typically has been observed in the 204 °C to 399 °C (400 °F to 750 °F)temperature range although corrosion which exhibits naphthenic acid characteristics has been reportedoutside this temperature range. Above 399 °C (750 °F), the naphthenic acids either break down or distillinto the vapor phase. While sulfidation will occur in both liquid and vapor phases, naphthenic acidcorrosion occurs only where liquid phase is present.e) The materials most vulnerable to naphthenic acid corrosion are carbon steel and the iron-chrome (5 to 12 %Cr) alloys commonly used in corrosive refining services. 12Cr may experience corrosion rates greater thanthat of carbon steel. Type 304 stainless steel offers some resistance to naphthenic acid corrosion at loweracid levels, but normally the molybdenum containing austenitic stainless steels (Type 316 or Type 317 SS)are required for resistance to greater acid concentrations. It has been found that a minimum Mo content of2.5 % is required in Type 316 SS to provide the best resistance to naphthenic acids.f) The amount of naphthenic acid present is most commonly indicated by a neutralization number or totalacid number (TAN). The various acids which comprise the naphthenic acid family can have distinctlydifferent corrosivities. The TAN is determined by an ASTM standard titration and is reported in mgKOH/g, which is the amount of potassium hydroxide (KOH) required to neutralize the acidity of one

2.B-10 API RECOMMENDED PRACTICE 581

2.B.2.6

Figures

Start

No

Is the

material C.S. or

300 Series

S.S.?

Yes

Yes

Do you know

the pH?

No

• Max. process

temperature

• Material

Determine corrosion

rate for Carbon Steel

and 300 Series SS

using Tables 2.B.2.3

and 2.B.2.4.

Determine pH

using Table

2.B.2.2

Cl Concentration

pH

Estimated

Corrosion Rate

Do you know

the Cl concentration of

the Acidic Water?

No

Determine Chloride

concentration from

Table 2.B.2.2.

Yes

Is the material Alloy

400 or Alloy B-2?

No

Yes

• Material

• Cl concentration

• Temperature

• Oxygen/Oxidant Present?

Determine

Corrosion Rate

using Table

2.B.2.6.

Determine

Corrosion Rate

using Table

2.B.2.5.

• Material

• Cl concentration

• Temperature

Estimated

Corrosion Rate

Estimated

Corrosion Rate

Figure 2.B.2.1—HCl Corrosion—Determination of Corrosion Rate

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