Hexavalent Chromium Risks for Fossil

and CCGT Plants

Sources, Sinks, and Risk Management
(plus chemistry)

ABHUG 2019
David Addison
david.addison@thermalchemistry.com
30/31 October and 1st November 2019
Revision 1.3

Includes technical information from

Nick Sarkis - ALS
 Chromium and Hexavalent Chromium
• Chromium common alloy element – stainless steels and high chromium containing materials.
Present in
– Steam turbines
– Gas turbines
– Boilers
– HRSGs

• Transition metal

• Multiple oxidation states

• Chromium III (3+) essential for human health (trivalent chromium)

• Stainless steel has oxide of chromium(III) oxide – normal product of corrosion

• Chromium VI (6+) solids extremely toxic (Hexavalent chromium) – often “brightly colored – yellow)
 Hexavalent Chromium
• Hexavalent chromium will pair with mono or divalent
cations but with the chromium in the VI (6+) oxidation
state e.g.

• Na2CrO4 – Sodium chromate

• CaCrO4 – Calcium chromate
• K2CrO4 – Potassium chromate

• Often also as a dihydrate with additional water of

crystallization present

• Used as industrial dye, leather tanning, wood

preservation, anti corrosion coating etc and formed
from stainless steel welding/plasma cutting etc
 Hexavalent Chromium Toxicity
• Carcinogen;
– Inhalation (breathing) - fumes or aerosols – increased rates of lung cancer
– Ingested (eating/mucus membranes) – eyes, nose, mouth – can cause permanent eye damage
– Dermal absorption (skin) – “chrome ulcers” + contact dermatitis

• Inhalation of Cr(VI) particulates can also cause

– Chronic respiratory irritation
– Chronic bronchitis
– Emphysema
• “Welders Cough”

• OSHA & similar permissible exposure limits examples for calcium chromate (100% solution)
– Inhalation – 8 hours at 0.05 mg/m3
– Dermal – 10% of body surface exposure has been reported as fatal
– Ingestion – ~0.5 g can result in serious toxicity
– Median Lethal dose – 50-150 mg/kg of body weight

• Easily managed risks with standard industrial hygiene and PPE strategies applied combined with neutralization where
needed

• Exposure can be detected by urine testing – pre and post testing – limit of concern is 5 µg/L. A change pre/post shift of
more than 10 µg/L or 25 µg/L at end of work week indication of excessive exposure. ½ life is 15-41 hours post exposure.
Levels relate to 8 hours at 0.05 mg/m3 exposure limit
 Hexavalent Chromium
• Risk associated with welding of high chromium materials well understood in industry

• Under high temp/energy welding hexavalent chromium fumes (chromium (VI) trioxide,
CrO3) produced under welding conditions

• Risks managed by
– Area ventilation – welding booths
– Masks/Supplied Air
– Skin protection
– Personal monitoring

• Can be present in boiler/HRSG chemical cleaning waste as well – depends on clean

chemistry. Well understood and managed as part of chemical clean procedures
 Gas Turbines/Steam Turbines/HRSGs
• Recently hexavalent chromium identified (but not full ionic structure reported) on
– Gas turbine hot gas path components
– Steam turbine hot external components - bolts
– HRSG/Boiler hot pipe external surfaces

• Safety notices issued by GT/ST OEMs (GE and Siemens) and some plant
owner/operators

• For GT/ST link made to calcium containing anti-size pastes used on hot components –
advised to avoid these products

• Can be tested for in the field via 3M test field test kits – fast screening tool

• May have been historically misidentified as sulfur deposits from fuel (also yellow but
thermodynamically impossible to be present)
 Gas Turbines

Rotor Bolts
GT Combustor Basket Spring Clips

GT Combustor Basket Flange

Steam Pipes

RH Bypass
HRSG Internal Surfaces – Access Doors
Hexavalent Chromium Safety Precautions - Example
 3M Chromate Test Kit – Field Screening

• Sensitive field test – color change

• Reported detection limit ~ 0.037 µg/L (ppb) – very low
• Only detects chromium (VI) not chromium (III)
3M Chromate Test Kit – Field Screening
 What is Going On Chemically?
• To get Hexavalent Chromium you need the following

1. A chrome containing material (will not occur with carbon steel)

2. A high temperature environment (in excess of 150-200+℃)
3. A oxidizing environment (%’s oxygen required, not ppb’s)
4. Chromium (III) oxide
– Oxidation of a chromium containing material or
– Corrosion of a chromium containing material
5. For a solid deposit (more to follow on this) you also need a specific salt/cation as a
counter ion to balance the reaction

Will not occur on water/steam touched surfaces. Gas pass or hot external
surfaces only under certain, specific conditions
 Hexavalent Chromium Form

• OEM data does not specify what chemical form the

Hexavalent Chromium is in

• Thermal Chemistry XRD/XRF testing however has for HRSG

casing deposits (NOTE: Only ever seen in one plant)

• Confirmed as Calcium Chromate or CaCrO4 – Yellow color

• GT/ST/Steam Pipe examples expected to also be calcium

chromate based on expected chemistry and physical
appearance.
HRSG Internal Surfaces – Access Doors – Stainless Steel
Liner Plates/Doors Only
HRSG Casing Deposit Analysis

Calcium
Chromate – 29% +
other calcium
containing salts
Chromium Corrosion Products

22% calcium, and 14%

chromium in deposit
 CHEMISTRY TIME - Hexavalent Chromium Formation
• Chromium oxide forms as a stable oxide on exposure to air or from corrosion
of chromium containing metals

– Cr2O3 - Chromium in III oxidation state – trivalent, non toxic

– Melting point = 2,435 ℃

– Boiling point = 4,000 ℃

– Once formed – should just remain on metal surfaces

– Trivalent chromium (III) essential trace mineral in diets – lack of leads to

“chromium deficiency”
 CHEMISTRY TIME - Hexavalent Chromium Formation
• However – take chromium oxide and add heat, oxygen and a source of calcium
following reaction occurs;

• 2Cr2O3 (Chromium (III) Oxide) + 3O2 + 4CaO (Calcium Oxide) = 4CaCrO4 (Calcium
Chromate (VI))

• Calcium chromate is a ionic solid, will not enter the gas phase and remains as a solid
on the surface of the material (2,710 ℃ melting point). The calcium acts as a
“chromate trap” keeping it on metal surfaces

• Calcium oxide is in some anti-seize pastes and also in insulation/lagging

• Calcium fluoride also in some pastes but no reaction occurs – Calcium+fluoride bonds
too strong to break
 Calcium Oxide Sources

• For GT/ST – if anti-seize paste used that contains calcium oxide then
reaction will occur at temperature if oxygen is also present

• For steam lines and HRSG cladding need water to move/leach insulation
calcium oxide to high temperature metal surfaces with oxygen present

• HRSG casing – rain water leaks online or during outages followed by

corrosion/reactions

• For steam lines – rain water lagging penetration likely during outages
followed by corrosion/reactions
 Anti-Seize Pastes

• CaO and CaF formulations (with graphite) developed as high temperature,

metal free (no nickel) anti-seize pastes

• Other formulations are metal based

• Key aspect is presence of CaO

• Excessive application of pastes likely exacerbates problems (the more CaO

available the more CaCrO4 able to be formed)
 Risk Management
• Hazard management advice from gas turbine and steam turbine OEMs technically etc
correct

• Key is to avoid contact – skin, mouth, eyes, lungs – standard industrial hygiene actions
should apply
– Try not to disturb/turn into a dust – can spray with oil (note still exists as hexavalent
chromium)
– P2 dust mask or full face mask or supplied air mask
– Nitrile gloves (plus barrier cream)
– Overalls/Disposable overalls
– Eye protection
– Clean up before food/drink ingress
– Any contaminated material should be treated as hazardous waste and disposed of
correctly or decontaminated* (see later)

• Deposit removal should be done with care to avoid dusts and skin contact
 Risk Elimination
• For GT/ST – if possible do not use calcium oxide containing anti-seize pastes in high
temperature and oxygen environments or do not over apply

• For HRSG casings critical aspect is to prevent rain water ingress that allows calcium
oxide leaching onto gas side surfaces with high chromium liner materials
– Main gas pass/doors
– Roof spaces
– Lower crawl spaces

• However – need to assume insulation side of liner plates (GT exhaust duct) have
calcium chromate present on them due to moisture + insulation contact

Bright yellow HRSG gas side deposits should be considered a major warning
sign and treated with significant caution
 Risk Elimination

• High energy chromium containing pipe work

– Need to eliminate/minimize rain water
ingress offline (control CUI) on at risk
materials
– For bolts etc - do not use calcium oxide
containing anti-seize pastes in high
temperature and oxygen environments

• Consider non calcium containing insulation for

these locations
 Calcium Chromate Removal/Decontamination
• Addition of a chemical reducing agent will convert Chromate (VI) to trivalent (III)
chromium – chromium (III) oxide (Cr2O3)

• Siemens current technical advice is to spray deposits of hexavalent chromium

with a ascorbic acid (C6H8O6 or HC6H7O6) + citric acid (C6H8O7) solution
(~10%) combined with some surfactant (for surface wetting)

• Likely possible reactions is as follows

12 CaCrO4 (VI) + 9 C6H8C6 = 12 CaCO3 + 6 Cr2O3 (III) + 10 C6H6C6* + 6 H2O

*ascorbic acid oxidises to dehydroascorbic acid – can be used as a oxygen

scavenger

• Residue (chromium (III) oxide (Cr2O3)) can then be rinsed away with demin
water. Reaction reported to take 10-15 mins until negative field test result
 Calcium Chromate Removal/Decontamination
• Reports from field indicate it works and is a easy and effective solution to
chemically converting the at risk form of chromium (VI) to the less toxic form of
chromium (III)

• Could be considered as a HRSG/steam pipe deposit spray which is then

cleaned up to prevent any other corrosion of surfaces from residual weak acid
solutions

• Could also be used to decontaminate clothing (soak overalls etc in bucket of

solution + then rinse with clean water) or equipment but be aware of weak acid
solution causing potential corrosion/damage to materials (harness etc)

• Ascorbic acid is a weak acid (pKa = 4.2, carboxylic acid) so similar to vinegar
(acetic acid - CH3COOH) and is easily oxidised
 Unconfirmed Risk Areas
• Review of historical inspection photos have identified other possible, likely calcium
chromate HRSG locations

• Upper and lower crawl spaces with;

– High chromium pipework
– High chromium liner plates
– Oxygen atmospheres
– High temperatures
– Calcium oxide containing insulation
– Potential for rain water ingress that allows for calcium leaching
• For HRSGs
– SH/Evaporator upper and lower crawl spaces
– GT Exhaust duct work – insulation side of plates mainly
– No sampling and confirmed laboratory testing to date (underway)
Lower Forward HRSG Crawl Space – Yellow/White Deposits
Upper Forward HRSG Crawl Space – Yellow/White Deposits
GT Exhaust Duct Plates – Insulation side of
plates
Note ascorbic acid
decontamination
underway
GT Exhaust to HRSG Transition – after rain water ingress
 Summary
• Hexavalent chromium on gas path surfaces that contain chromium (cannot
get with carbon steel) is another risk associated with power plants – has
always been present in plants with welding and chemical cleaning wastes
and similar, robust, effective management strategies apply to minimize
health and safety risks

• Hexavalent chromium is a known toxic carcinogen/health risk and

associated risks need to be managed carefully at all plants with proven,
effective and standard industrial hygiene techniques – no reasons why
effective and safe management cannot be carried out

• Risk management is well understood from welding and industrial handling of

chromate materials – key is to avoid contact and this can be safely done during
outages etc with sufficient planning, equipment and PPE without a major impact
on work flows etc
 Summary
• Discontinuation or limiting use of calcium oxide containing anti-seize pastes for
GT and ST application will significantly lower risks as without calcium, calcium
chromate cannot be formed

• Field testing of hexavalent chromium can be done with commercial test kits
easily – very low detection limit/sensitivity

• Reaction with 10% ascorbic acid solutions will chemically reduce

hexavalent chromium (VI) to the less toxic trivalent chromium (III) and
removes the risk. Solutions can also be used for decontaminating
equipment etc (note weak acid solution)

• Minimization/elimination of water leaching of calcium oxide from insulation onto

high chromium pipes and HRSG gas path surfaces will lower risks
 Hexavalent Chromium Risks for Fossil
and CCGT Plants
Sources, Sinks, and Risk Management
(plus chemistry)

ABHUG 2019

David Addison
david.addison@thermalchemistry.com
30/31 October and 1st November 2019
Revision 1.3