CORROSION – IT’S ABOUT TIME

MetCorr-SES

Bruce Craig

MetCorr-Stress Eng Services

February 7, 2017

ASM Houston Seminar – Materials Selection and

Corrosion Management in Oil Field Environments

MONTAIGNE’S AXIOM

Nothing is so firmly believed

as what we least know.

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CORROSION RATE

We measure corrosion rates and predict them:

Using values like mpy which denote uniform thinning.

Crack thresholds and crack growth rates for

environmental cracking.

We then take such numbers to design equipment

or predict remaining life but is this adequate?

Is it correct?

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WHAT WE REALLY MEAN BY CORROSION RATE IS –

HOW LONG WILL IT LAST?

IT’S ABOUT TIME NOT RATE AND MOST OFTEN WE

DON’T HAVE A GOOD ANSWER.

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GENERAL/UNIFORM CORROSION

We have this down pretty good.

For example, CO2 corrosion models generally

assume uniform corrosion but as stated in a

review of models, “some CO2 models include

a “qualitative” risk for localized corrosion

attack” (Nyborg, Corrosion 2010, Paper 10371).

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CO2 CORROSION MODELS ARE QUITE GOOD

Results for tubing corrosion

prediction in the Haynesville

Shale.

Predicted TTF ~ 5 months.

Actual TTF = 6 months

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BUT WE CAN’T ALWAYS GET THE

MESSAGE ACROSS!

Email from Pipe Supplier

We sold a customer 13Cr tubing until they switched to 2-3/8”,L-80 to save money.

The pipe was in these wells for one year, then failed from corrosion.

They now have more than 1,000,000 ft. of steel tubing in Haynesville Shale wells.

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WALL LOSS AND CORROSION ALLOWANCE

Modelling CO2 corrosion typically assumes uniform

wall loss then we adjust the CA to suit the desired life

time.

However, pitting (localized attack) is not linear and

there are no adequate models at present to evaluate

the usefulness of CA for pitting.

So how much time do we have (TTF) when pitting is

the mechanism?

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CRAs

Fail by Localized attack – pitting, EC

Currently no means to predict expected life of

CRAs so we are overly conservative.

PREN is frequently misapplied to downhole

oxygen free environments.

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FUNDAMENTAL ISSUES WITH PITTING AND EC

PREDICTION FROM A TIME PERSPECTIVE

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PITTING

The common practice in the oilfield is to use a multiplier such as 10x

the uniform metal loss.

However, the situation is far more complex and a simple rate is not

adequate to define the life of a component in service.

Pitting is a stochastic (probabilistic) process that involves;

Pit initiation (this includes an incubation time)

Metastable pitting with possible repassivation

Stable pitting and growth/ or death and arrest.

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SHALLOW PITS AND THE INITIATION OF

SUBCRITICAL EC

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Critical pitting potential and repassivation potential can be measured in the lab

but are not easily translated into actual field process conditions that can be

monitored for pitting prediction.

Extreme value methods (ie Gumbel) can be used for direct measurement of

existing pits to predict pit growth rates but not pit initiation which for CRAs is more

important.

Since there are no ready accurate models for pitting of CRAs we make very

conservative choices for their application to avoid any pitting and EC.

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PITTING AND EC ALWAYS HAVE AN INCUBATION TIME

This is what we need to focus on – time to initiate

attack.

Complete resistance would = ∞ incubation time.

Poor resistance would = seconds/minutes/hours/days

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ENVIRONMENTAL CRACKING

HSC (HE) – Fast

(A common question is how long will Q125 last

when exposed to H2S in my well?)

SCC – Slow

(How slow is slow? Depends on many factors including

inspection interval)

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SCC – WHEN WILL CRACKS ALL CONNECT?

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HOW LONG DO WE HAVE BEFORE LINKAGE?

18HIC in Plate Steel Exposed to Sour Gas

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“One of the toughest thresholds to dispel is KISCC”.

“The incorrect concept of SCC thresholds may be

related to the desire of designers and engineers to

identify conditions that will permit the implications

of SCC to simply be ignored.”

Peter Andresen (recognized authority on SCC) in Proc. Of

Symposium on Chemistry and Electrochemistry of Corrosion and

Stress Corrosion Cracking.

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BUT STILL WE PERSIST-API 17TR8

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• H controlled Kth and [da/dt]II can be predicted for some alloys from mechanism-based understandings and limited experiments. However, this is contingent on knowledge of:Mechanics of crack tips and material & environment dependent fracture

mechanismsMaterial-dependent hydrogen-metal interactions and H transport ratesMaterial, geometry and chemistry dependent crack tip hydrogen

concentrations

• Many parameters remain elusive or difficult to determine at the correct length scale but new techniques may enable such determination. (Prof. John Scully, UVA).

• In other words you can’t simply take KISSC from FM tests and stick it into LEFM equations!!!

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HYDROGEN STRESS CRACKING (aka HE)

For thin wall components (and even many thicker ones) ie. casing

steels, pipeline steels, etc. the idea of crack growth rate is

meaningless.

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THE IMPORTANT FACTOR IS CRACK INITIATION NOT

PROPAGATION.

Most well tubulars are < 1” wall as are

pipelines.

In H2S at even 10 inch/min that means the

crack will cross the entire wall in 6 minutes!

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HYDROGEN DIFFUSION AND TRAPPING

H

H

H

H

H+

H+

H+

H+

H+

H+

H+

H+

• Casing ID• Riser ID• PL ID

Hydrogen gradient

Stress

Pipe OD

It takes time to

accumulate Hcr

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DSM WELDS (NO PREEXISTING FLAWS)

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DSM WELDS (NO PREEXISTING FLAWS)(BUT FOLKS DRIVE THEMSELVES CRAZY LOOKING FOR A PREEXISTING DEFECT!)

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API FULL SCALE CASING SSC TEST

W/PRE-EXISTING FLAWS

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CRACK INITIATION, SMOOTH SURFACES AND

INCUBATION TIME

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INCUBATION TIMEMetCorr-SES

Elsea and FletcherDMIC 1964

CRACK INITIATION

Crack initiation at the bottom

of a notch in 304 SS at surface

instability.

sinusoidal surface

profile due to wave

growth induced by an

applied stress, σ.

(Asaro-Tiller)

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NACE MR0175/ISO 15156

P110 Casing in Sour Shale Wells

NACE says one factor to consider is time of exposure but how?

• Millions of feet of P110 casing in sour Eagle Ford and Haynesville fields with very few failures due to SSC.

• NACE MR0175/ISO 15156 prohibits P110 since it is prone to failure from SSC.

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FAILURE OF AN EARLY PRODUCTION RISER

(HE IN CO2 w/o H2S)

Sufficient hydrogen from the CO2 corrosion reaction plus zinc

thread dope.

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STRESS CORROSION CRACKING

Fast or slow cracking

SCC of pipeline steels – slow

HIC – slow

But the important factor for both of these is

branched crack linkage which can’t be

predicted.

Therefore, crack initiation is the key.

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THE FUTURE – MULTISCALE MODELING (MSM)

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Overly simplistic LEFM and pitting factors

will not and do not work.

AtomsCrystal

StructureMicrostructure Component

Thermo-mechanical

Treatments

Phases Defects+

• Vacancies

• Dislocations

• Twins

• Stacking Faults

• Grain Boundaries

• Voids

• Cracks

& their distribution

Travel across length scales to reach the scale of the component

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YS

KI

Etc.

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Unit Cell* Crystalline Defects Microstructure Component

*Simple Unit Cells

AngstromsDislocation Stress fields

→ NanometersMicrons Centimeters

Grain Size

Lengthscales in Materials Science

Start with a cursory look at the length scales involved in Materials Science

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MSM OF PIT INITIATION

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Herbert et. al.,

Electrochimica Acta, 2014

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HE modeling using

Quasi-Continuum

Density Function

Theory (QCDFT),

quantum mechanics

(QM) and molecular

dynamics (MD).

Zhang and Lu

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CONCLUSIONS

Simply modeling uniform corrosion (wall loss), pit propagation and crack growth rates are not sufficient for future oil and gas projects.

In fact these approaches are non-conservative and may be dangerously misleading.

In thin wall components and for HSC, initiation of pitting and cracking are of paramount importance -not propagation.

In other words its about time – incubation time.

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DEDICATION

Dedicated to two good friends and corrosion engineers that passed away last year.

Don Drake and Tom Bednarowicz