good bugs, bad bugs; sol-gel encapsulated bacteria in anti- fouling and anti-corrosion coatings...

Good bugs, Bad bugs; Sol-gel Encapsulated Bacteria in Anti-Fouling and Anti-Corrosion Coatings

Professor R. Akid & Dr H. WangCentre for Corrosion Technology

r.akid@shu.ac.uk

Dr T. J. SmithBiomedical Research Centre

t.j.smith@shu.ac.uk

Sheffield Hallam University

What are the benefits of these coatings?

For industrialised countries the cost of Corrosion is currently around 3-4% GDP. This is estimated this at a cost of $140Bn

Bridges, railroads Gas, Electricity distribution

Road, air , sea

Oil & gas, chemicals

DefenceNuclear waste

1 Hurricane Katrina every year!

Costs of fouling

• 1994 – world shipping fleet burnt 184 Million tonnes of fuel oil.

• If no antifouling paints are used this fuel consumption is increased by 40% (= 72M tonnes of FO)

• Note in that year the North Sea oil platforms produced 100M tonnes of FO

Existing antifouling/corrosion strategies

• Use of inhibitors and biocides

• – Expensive

• – Often ineffective

(location & concentration issues)

• – Can be damaging to the environment

Outline• Sol-gel : Materials chemistry and

anti-corrosion aspects (RA)

• Sol-gel : Microbiology and Antifouling aspects (TJS)

• Summary

• Acknowledgements

Gelation is the process of bond formation

Evaporation

Gelation

GelSolNanocompositedense material

Cure

at T & t

Formation of Sol-gel MaterialsFormation of Sol-gel MaterialsWhat is sol-gel?

A sol is a colloidal suspension of solid particles (1-1000nm size) in a liquid

What is a gel?

A gel is a substance that contains a continuous liquid phase

What is gelation?

Sol gel chemistrySol gel chemistry

Metal substrate

Precursor Si (OC2H5)3 = Si-O-R', where R' = C2H5

O

R'-O-Si-O-R'

Si-O-R'

Si

O

O

Hydrolysis

Condensation

Tetraalkoxysilanes – (Methoxy or Ethoxy)

Bond Formation of the Sol-gel CoatingBond Formation of the Sol-gel Coating

Metal

Sol-gel applied onInner layer

Outer layer

-O-Si-O-R'

Si-O-R'

Si

O

O

O

MO ─

Si

OR

M

Si O

O

Sol gel Al

interface

Si particles

Opportunities for organic-inorganic hybrid sol-gel Opportunities for organic-inorganic hybrid sol-gel basic network structuresbasic network structures

1. Modify the Si backbone

3. Modify silicon structure with functional organic groups (R)

2. Incorporate three-dimensional inorganic oxide network based on silicon or other metals ( M= Ti, Zr, or Al)

3. Encapsulated functional additives, e.g., bacteria, antibiotics, inhibitors

M

Sol gel Application MethodologySol gel Application Methodology

Cure at selected

temperature

Apply top coat directly to sol gelfor anti-corrosion coating

Use asfunctional/barrier coating

Colloid solution(s)Organic and

Inorganic components

Functional Additivese.g., corrosion inhibitors,bio-active molecules, etc.

Mix and Age*

*Ageing time dependant upon formulation chemistry

Apply to

metal;

Dip, Spray..

Bioactive coating for Bioactive coating for anti-fouling and anti-microbial anti-fouling and anti-microbial

induced corrosion applications.induced corrosion applications.

Background

• Fouling & Microbially-induced corrosion– Marine corrosion is exacerbated by the

formation of destructive biofilms on metal surfaces

– For example, sulfate-reducing bacteria (SRB) such as Desulfovibrio desulficurans forms H2S as a metabolic by product

Microbiologically Influenced Corrosion (MIC)(Bacteria & Biofilms)

}

Colonisation ofSulphateReducingBacteria(SRB)

H2S formation

Localised Corrosion(pitting)

Microorganisms, especially bacteria, colonise surfaces to form Biofilms

Biofilm formation; up to 48hrs depending upon temperature

Consequences of MICConsequences of MIC

Current Approaches to mitigate Fouling & MIC

• Application of synthetic polymers/paints: some bacteria can use the coating as a hydrocarbon food source

• Controlled dosing with biocides: impacts upon the environment

• Changes in environmental conditions, e.g., remove water from fuels, oils etc. not often feasible

Biocoat approach• Bacteria can reduce corrosion• Coating designed upon fundamental knowledge of

corrosion and microbial ecology

Do protective bacteria exist and work?

High

Corrosion Rate

Low

Note: the bacterial strain(s) are added as planktonic bacteria (i.e., freely suspended)

Antifouling/MIC approach at SHUAntifouling/MIC approach at SHU

• Combination of anti-corrosion sol-gel coating and protective bacteria.

• Uniform distribution of protective bacteria fixed on the surface

Substrate

'Biocoat'

Viable bacterial cells immobilised in coating

Paenibacillus polymyxa

• A bacterium that actually inhibits corrosion and biofouling

• often found in soil• non-pathogenic• Forms highly-resistant

endospores in response to environmental stress

• Endospores remain inert until nutrients/germinants available

Magnification x 1000

Paenibacillus polymyxa endospores

Viability of P. polymyxa endospores within sol-gel coating

Viability of P. polymyxa endospores within sol-gel coating on AA 2024 T3

• Following immersion in artificial sea-water, germination occurs, forming microcolonies within the sol-gel microstructure

• Coating thickness ~10µm

• Akid R, Wang H, Smith T. J, Greenfield D, and Earthman, J. C, 2008, Advanced Functional Materials 18, 203-211

Magnification x 1000

Abiotic

Biotic

Colonisation of cells within sol-gel coatingColonisation of cells within sol-gel coating

Rods - Vegetative cells

Solid discs - Endospores

Immersion in nutrient broth for 1 hour

Immersion in nutrient broth for Immersion in nutrient broth for 8 hours8 hours

Spores in the coating remain viable

• There is an increase in the number of vegetative cells visible under fluorescence microscopy the longer the Al 2024 coupons are immersed in the nutrient broth

• This suggests a sustained ability of the spores to germinate under these conditions, and that enough nutrition is able to reach the spores in order to induce germination

Propagation of corrosion/biofouling bacteria from the coating

• It was possible to recover vegetative cells from the nutrient broth, following removal of the metal substrate

• This indicates the release of vegetative cells from the sol-gel coating that are the result of the germination of encapsulated spores

Bio-active coating - field trialsBio-active coating - field trials