Zinc and zinc alloy plating

Erik Galdames

Bach. of Eng.

E-46800 Xativa (Spain)

Content

• Electroplating fundamentals

• Zinc plating

• Zinc plating coating properties

• Zinc plating hardware

• Zinc plating flow-chart

• Zinc plating process parameters

• Zinc plating process control

• Zinc alloy plating

• Zinc-nickel plating flow-chart

• Zinc-nickel process parameters

• Zinc-nickel process control

• Zinc and zinc-nickel plating performance

• Zinc and zinc alloy performance

• Faraday’s law: m = electrodeposited mass (g): I = Current applied (A), t = time (s), M = Atomic weight (g) n = interchanged electrons,

• Faraday’s law for thickness e = electrodeposited thickness (µm) (g), j = current density (A/dm2), t = time (s), n = interchanged electrons, d = element density (g/cm3).

• Theoretical performance. It is the expected performance according to Faraday’s law expressed in percentage. Theoretical performance 100%

• Actual performance. It is the real performance taking into account physical constraints (conductivity of the solution, heat losses, etc.) Actual performance < 100%

• Electrodeposition rate (µm/min.): Thickness of electrodeposited layer per unit of time

.

Electroplating. Fundamentals

Electroplating. Fundamentals

Electroplating. Fundamentals

FACTORS WITH AN INFLUENCE IN ELECTROPLATING

• Part geometry

• Position of anodes

• Position of cathodes

• Ratio cathode/anode

• Nature and surface condition of part

• Current density

• Agitation of electrolyte (bath)

• Temperature

• Concentration of metallic ions

• Concentration of hydrogen ions (pH)

• Additives (brighteners, surfactants, etc.)

Electroplating. Fundamentals

Current density

– The higher the current density, the higher the electrodeposited layer thickness

– Size of crystal formed is reduced, provided that current density is within limits

– If current density increases beyond practical limits, a more rugous film will be electrodeposited

Temperature

– Temperature generally favours electroplating processes. Less hydrogen is diffused on the cathode (the part to be coated)

– If temperature and current density both increase, brighter deposits will be obtained

– If temperature increases and current density is not changed, the size of metallic crystals will increase and consequently, rugosity of film will increase

– The higher the temperature, the higher the consumption of brighteners and other additives in the bath

Metallic concentration

– If the metal concentration is low, brighter deposits with fine crystals can be obtained. If concentration is too low, layer thickness will be minimum

– If the metal concentration is too high, layers of higher roughness will be obtained

Electroplating. Fundamentals

Electroplated finishes are coatings of pure or allied metals by means of DC

It is the reversed process produced in a battery. Electricity is generated by means of ionic dissolution of metals in a battery

In the case of electroplating processes metals are deposited from a ionic solution by means of an electric current

Electroplating processes follow Faraday’s law

– The quantity of an electrodeposited coating depends of the metal to be electrodeposited (chemical equivalent)

– The higher the current, the higher the quantity of electrodeposited metal

– The larger the surface area to plate, the lower the quantity of electrodeposited metal

– The longer the electroplating process, the higher the quantity of electrodeposited metal

Other factors

– The higher the conductivity of the bath, the higher the quantity of electrodeposited metal. Type of electrolyte (acidic, alkaline)

– Temperature influences in the rate of metal electrodeposition

– Cleanliness of surface of the metal to be plated

– Content of impurities on the surface of the metal to be plated

Zinc plating

Electroplated zinc is widely used for coating steel parts for corrosion protection

Zinc offers cathodic protection to steel due to a redox reaction. Zinc is rusted to protect steel. As far as zinc is present, there will be cathodic protection against corrosion

If the zinc layer has cracks, steel will still be protected against corrosion

This phenomenon does not occur with paints or with nickel and copper plating where surface cracks or discontinuities allow corrosion to penetrate below the surface and extend the corrosion under the coated surface and corrodes the base mteal

Zinc plating offers temporary corrosion protection

Zinc also can be deposited in form of alloy with other metals such as nickel, iron, cobalt and tin

Zinc layers can be treated with chromic passivations and sealers, which increase corrosion protection substantially (white and red rust). Zinc layers also allow further lubrication, which changes the friction behaviour of metallic fasteners

Nowadays, most of the passivation products do not contain Cr6+

Zinc plating

Zinc layers can be treated with chromic passivations and sealers, which increase corrosion protection substantially (corrosion resistance to white and red rust). Lubrication with liquid or solid lubricants is also possible. Lubrication changes the friction behaviour of the electrodeposited surface completely, thus the properties of the coated part, especially in a metallic threaded fastener

Some years ago, it was common to see chromic formulations with Cr6+ for passivation processes. Cr6+ is carcinogenic and has been prohibited for use in many industries. These formulas were known as chromating or bichromating. Typical colours seen were:

– Yellow to bronze. Iridescent yellow

– Olive green

– Black

Cr6+ was widely used by its corrosion protective properties, since it had self-healing effect, i.e. sBadl scratches or superficial damage at microscopically level were self-repaired and corrosion protection was maintained.

Due to restrictions of use of Cr6+, the majority of passivation products are made of formulas without Cr6+ (Cr6-free processes). They contain Cr3+ mainly. Typical colours:

– Transparent or clear, silver colour or bluish. Normally thin layer passivation

– Transparent iridescent with reddish, yellowish or bluish nuances. Known as thick layer passivation

– Yellow to bronze, iridescent yellow (not present in Europe)

– Black (not widely used)

Cr3+ provides no self-healing effect and needs sealers or top-coats to increase corrosion protection

Zinc plating

Zn + transparent

passivation

Zn + transparent

iridescent passivation

Zinc plating

Surface preparation before coating mainly is: – Chemical cleaning – Rinse – Acid pickling – Rinse – Anodic electrocleaning – Rinse – Acid activation (not used for acid zinc plating) – Rinse

Zinc plating has the following types of electrolyte – Acid zinc plating – Alkaline zinc plating – Alkaline cyanide *

Post-treatments – Rinse – Baking (for high tensile bolts > 10.9) – Passivation (optional) – Rinse – Top-coats, sealers or lubrication (optional)

– *Not used anymore

Zinc plating

Acid zinc plating Widely used technology

High plating efficiency > 90%

Less quantity of H2 generated

Faster electrodeposition

Excellent covering power

Poor throwing power

Bright deposit, special for decoration purposes

Less permeable to hydrogen effusion

Poor thickness distribution

Poor ductility

Zinc plating

Alkaline zinc plating Widely used technology

Good plating efficiency > 60%

More quantity of H2 generated

Medium electrodepositing rate

Less brighter deposit

Good covering power

Medium throwing power

More permeable to hydrogen effusion

Good thickness distribution

Good ductility

Zinc plating coating properties

Layer thickness distribution

The longer the part, the higher the

differences in layer thickness between high

current /low current density areas

Recess areas are low current density

areas. Layer thickness is lower

Uneven coating layer thickness in low

current density areas wiith acid zinc plating.

Recess areas with poor layer thickness

Acid zinc plating Alkaline zinc plating

20 µm 5 µm

Long part Small part

5 µm 10 µm

8 µm 15 µm

Zinc plating technology

Barrel zinc plating For small parts Bulk process Widely used and automated technology Good productivity Low labour cost Reproducible results

Rack zinc plating For large parts Parts fixed on racks Widely used and almost totally automated technology Good productivity Higher labour cost if loading not automated Excellent appearance of coated parts Reproducible results, but effect of low/high current density

areas must be taken into consideration

Zinc plating hardware

Zinc plating line on barrel

Zinc plating hardware

Zinc plating barrel

Zinc plating hardware

Zinc plating line on rack

Zinc plating hardware

Lay-out of a zinc plating line

RIN

SE

CL

EA

NIN

G

RIN

SE

AC

ID P

ICK

LIN

G

RIN

SE

EL

EC

TR

OC

LE

AN

ING

RIN

SE

RIN

SE

RIN

SE

AC

ID A

CT

IVA

TIO

N

RIN

SE

RIN

SE

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

ZIN

C P

LA

TIN

G

RIN

SE

RIN

SE

RIN

SE

AC

ID A

CT

IVA

TIO

N

RIN

SE

PA

SS

IVA

TIO

N

RIN

SE

RIN

SE

RIN

SE

CO

LD

DR

YIN

G

SP

INN

ING

TO

P-C

OA

T

HO

T A

IR D

RY

ING

HYDROGEN EMBRITLEMENT RELIEF OVEN

Typical lay-out of a conventional

zinc plating line on barrel.

Design may vary and may

include other additional baths or

plating stations to increase

production (up to 20 plating

stations)

Hydrogen embrittlement relief is

done just after plating.

Passivation is applied after

hydrogen embrittlement relief

Zinc plating hardware

Lay-out of a zinc plating line

Every plating station has its own

rectifier. An ancillary tank is

used for all stations to provide

the necessary amount of Zn

ions. A filter is used to filtrate

the plating solution

Plating

station

Rectifier

Ancillary tank

for zinc anode

dissolution

Reservoir

Zinc plating flow-chart

CLEANING

2-STEP RINSE

ELECTROCLEANING

2-STEP RINSE

ACID ACTIVATION

RINSE

ZINC PLATING

RINSE

ACID PICKLING

2-STEP RINSE

2-STEP RINSE

COLD DRYING

TOP-COAT

DRYING

ACID ACTIVATION

PASSIVATION

COOL DOWN

DRYING

BAKING

Hydrogen

embrittlement

relief?

NO

YES

Zinc plating process parameters

Agent Temperature Process parameters Description

Cleaning Alkaline soak cleaner 50 to 60ºC

Removal of soils (oil, grease, machining residues)

Acid cleaning HCl or H2SO4 with acid

inhibitor

RT or 35ºC HCl = 10-25%

H2SO4 = 10 – 15%

Removal of rust

Electrocleaning Alkaline soak cleaner

applied under current

60ºC to 70ºC NaOH: 50 to 100 g/L

Current dens..: 5 to 10 A/dm2

Cleaning l of soils not removed by previous steps

(e.g. fingerprints,

Acid activation HCl RT 5% Activation of the surface prior to coating

Zinc plating

(alkaline non-

cyanide)

Zn

NaOH

Carriers

Brightener

20º to 28ºC Zn: 7 to 10 g/L

NaOH: 110 to 120 g/L

Current density: 1 to 2 A/dm2

Voltage: 5 to 10 V

t = 50 to 60 min.

Carriers and brightener acc. to

formulator

Plating

Baking -- 200ºC to 220ºC > 4 h (1 h max. within

electroplating and baking)

Removal of hydrogen introduced by acid pickling

and electroplating itself. Applicable to high-

strength fasteners P.C. ≥ 10.9 and 1000 N/mm2

Acid activation HNO3 RT 1% Previous step to passivation

Passivation Cr containing acidic

solutions

RT or

35ºC to 60ºC

Conc. depending on formulator

pH 1,3 to 2,5

Increase of corrosion protection (white rust

prevention), colour (silver, silver iridescent, black,

yellow

Top-coat Organic and mineral based

alkaline solution

Mainly RT, up to 50ºC Depending on formulator

pH slightly alkaline

Enhancement of corrosion protection of metallic

coatings. Integrated solid lubrication (optional)

Typical process parameters of zinc plating on barrel. They can be

different according to the needs of the plater

Zinc plating process control

Characteristics to be checked Concentration of cleaner, temperature, filtration and oil skimming

Concentration of acid pickling bath, pollutants such as Zn and Fe. Inhibitor performance > 90%

Anodic electrocleaning must be used for fasteners. Temperature

Concentration of Zn, NaOH of zinc plating bath

Temperature of zinc plating bath

Current density. Current and voltage of rectifiers

Plating time

Hull cell to control additives (carriers, brighteners)

Automated dosing of chemicals

Filtration of the zinc plating bath

Ratio cathode/anode

Alkaline vs. acid electrolyte for hydrogen embrittlement avoidance

Hydrogen embrittlement relief for high-strength fasteners mandatory (P.C. >10.9 bolts and parts with Rm ≥ 1000 N/mm2

Concentration of passivation, pH, pollutants (mainly Zn, Fe), temperature

Efficiency of rinsing operations. Concentration of pollutants. Use of flow-meters

Zinc alloy plating

Zinc allows its electrodeposition as an alloy with other metals such as cobalt, nickel, iron and tin

The most widely used are zinc-iron, zinc-nickel and also tin-zinc.

Zinc-iron. It consists of an alloy of zinc and iron with a content of Fe of 0,3 to 0,8% approximately. It allows Cr3-free black passivation and also transparent passivation, though black passivation is widely used. Medium corrosion protection. This finish is not longer preferred

Zinc-nickel. It consists of an alloy of zinc and nickel with a 12% to 15% Ni content. Excellent corrosion protection even on high temperature conditions (vehicle compartment applications). It allow Cr3-free transparent and black passivations. Good base for e-coat paints. High temperature resistance. High ductility after bending and crimping operations

Tin-zinc. It consists of an alloy of zinc and tin (Sn 70% Zn 30%). Excellent corrosion protection. Very good weldability and electrical conductivity

Zinc-Nickel plating flow-chart

CLEANING

2-STEP RINSE

ELECTROCLEANING

2-STEP RINSE

ACID ACTIVATION

RINSE

ZINC-NICKEL PLATING

RINSE

ACID PICKLING

2-STEP RINSE

2-STEP RINSE

COLD DRYING

TOP-COAT

DRYING

PASSIVATION

COOL DOWN

DRYING

BAKING

Hydrogen

embrittlement

relief?

NO

YES

Zinc-Nickel plating process parameters

Agent Temperature Process parameters Description

Cleaning Alkaline soak cleaner 50 to 60ºC

Removal of soils (oil, grease, machining

residues)

Acid cleaning HCl or H2SO4 with

acid inhibitor

RT or 35ºC HCl = 10-25%

H2SO4 = 10 – 15%

Removal of rust

Electrocleaning Alkaline soak cleaner

applied under current

60ºC to 70ºC NaOH: 50 to 100 g/L

c.d.: 5 to 10 A/dm2

Cleaning l of soils not removed by previous

steps (e.g. fingerprints,

Acid activation HCl RT 5% Activation of the surface prior to coating

Zinc-Nickel

plating

(alkaline)

Zn

NaOH

Carriers

Brightener

20º to 28ºC Zn: 8 to 11 g/L

Ni: 1,9 to 2,3

NaOH: 110 to 130 g/L

c.d.: 1 to 2 A/dm2

t = 120 to 160 min.

Plating

Baking -- 200ºC to 220ºC > 4 h (1 h max. within

electroplating and baking)

Removal of hydrogen introduced by acid

pickling and electroplating itself. Applicable to

high-strength fasteners P.C. ≥ 10.9 and 1000

N/mm2

Passivation Cr containing acidic

solutions

RT or

35ºC to 60ºC

Conc. depending on

formulator

pH 1,3 to 2,5, depending

on type of passivation

Increase of corrosion protection (white rust

prevention), colour (silver, silver iridescent,

black, yellow

Top-coat Organic and mineral

based alkaline solution

Mainly RT, up to

50ºC

Depending on formulator

pH slightly alkaline

Enhancement of corrosion protection of

metallic coatings. Integrated solid lubrication

(optional)

Typical process parameters of zinc-nickel plating on barrel. They

can be different according to the needs of the plater

Zinc-nickel plating process control

Characteristics to be checked Concentration of cleaner, temperature, filtration and oil skimming

Concentration of acid pickling bath, pollutants such as Zn and Fe. Inhibitor performance > 90%

Anodic electrocleaning must be used for fasteners. Temperature

Concentration of Zn, Ni, NaOH of zinc-nickel plating bath. Use of AAS (Atomic Absorption Sprectroscopy) for analysis of metal content

Temperature of zinc-nickel plating bath(use of heat exchangers)

Current density. Current and voltage of rectifiers

Plating time

Hull cell to control additives (carriers, brighteners)

Automated dosing of chemicals

Filtration of the plating bath

Ratio cathode/anode

Turnover (recycling of bath to maintain homogeneous concentration)

Removal of carbonates (chiller or membrane technology)

Alkaline vs. acid electrolyte for hydrogen embrittlement avoidance. Acid electrolyte for parts made of cast iron and high-carbon steel

Hydrogen embrittlement relief for high-strength fasteners mandatory (P.C. >10.9 bolts and parts with Rm ≥ 1000 N/mm2

Concentration of passivation, pH, pollutants (mainly Zn, Fe), temperature

Efficiency of rinsing operations. Concentration of pollutants. Use of flow-meters

Zinc and zinc-nickel plating performance

Zn Zn Zn ZnNi ZnNi ZnNi

Coating Zn+transp.

passivation

Zn+passiv.

transp.+sealer

Zn+thick layer

passivation

ZnNi+transp.

passivation

ZnNi+transp.

passivation

ZnNi+black

passivation

Layer thickness 8 µm 8 µm 8 µm 8 µm 8 µm 8 µm

NSS WR 24 120 h 72 h 96 h 120 h 96 h

NSS RR 144 h 240 h 144 h 720 h 720 h 720 h

Colour Silver Silver Iridescent yellow Silver Silver Black

Cr6+ No No No No No No

Hydrogen

embrittlement

risk

Yes Yes Yes Fair Fair Fair

Temp. Resistance 80ºC 100ºC 80ºC 150ºC 150ºC 150ºC

Multi-tightening NO Conditional No No Conditional

No

Sealing No Yes No No Yes Yes

Cost + ++ + ++++ ++++ ++++

Zinc and zinc alloy properties

Properties of zinc and zinc alloy coatings Alkaline zinc Alkaline zinc nickel Alkaline zinc-iron

Properties Appearance

Weldability

Wear resistance

Ductility

Good

Fair

Fair

Fair

Good

Fair

Good

Fair

Good

Fair

Fair

Fair

Corrosion protection

(as plated)

White rust

Red rust

Fair

Fair

Excellent

Excellent

Excellent

Good

Corrosion resistance

(after thermal

conditioning)

White rust

Red rust

Low

Low

Good

Good

Low

Low

Corrosion resistance

(after bending)

White rust

Red rust

Fair

Fair

Good

Good

Fair

Fair

Characteristics of

electrolyte

Distrib. (min./max)

Plating efficiency

Covering power

Bath control

Good

Fair

Fair

Good

Excellent

Low

Fair

Fair

Good

Fair

Fair

Fair

Analysis and

measurement

techniques

X-ray fluorescence

Magnetic induction

Coulometric

Good

Good

Good

Good

Bad

Good

Good

Fair

Good

Alloy content %

(X-ray fluorescence) -- Good --

Summary

Electroplated zinc provides cathodic protection to steel parts. It provide medium term corrosion protection

Zinc can also be electrodeposited as an alloy to provide better corrosion protection

Zinc-nickel and tin-zinc can provide excellent corrosion protection compared to pure zinc

Zinc-nickel is widely used in the automotive industry

Tin-zinc is used in welding applications and where conductivity is important. It is used in aerospace industry

Zn, ZnNi, SnZn provide good electrical conductivity

Good (pure and tin-zinc) to fair weldability (zinc-nickel)

Zn, ZnNi provide uniform coating distribution except on large parts (high-current density areas)

Risk of hydrogen embrittlement on high-strengh fasteners (≥ 10.9 bolts or parts with tensile strength > 1000 N/mm2). Necessary to apply hydrogen embrittlement relief processes