Galvanizing Bath Management

Introduction

Galvanizing Bath (GB) is the heart of the galvanizing process. It is preceded by cleaning / pickling/ annealing of strip & followed by Air / N2 / steam wiping, accelerated cooling, passivation treatment etc. in a Continuous Galvanizing Line (CGL). End properties/ performance of the galvanized product depend to a large extent on the control of the levels of various process parameters in a GB.

Bath Composition :

In any galvanizing/Zn-Al/Zn-Fe coating process the bath composition plays a prime role in deciding the

  1. Chemical composition of the coating
  2. Spangle morphology
  3. Coating Lustre / Brightness
  4. Coating Adhesion.
  5. Corrosion Resistance

Hence, designing the bath composition basically emanates from the desired composition of coating and its final property requirements. A galvanizing bath generally contains mainly Zn (>99%), small amounts of minor additives like Pb, Al, Ni etc. The bath may also contain some other elements like Sn, Sb, etc. as unavoidable impurities .

The influence of various additives (minor) & impurities on the operation of CGL and quality of the galvanised sheets are detailed below.

Aluminium

The major advantages of Al-addition are:

  1. Formation of Fe-Al interfacial layer: Inhibition Layer
  2. Suppression of Fe-Zn IMC Formation
  3. Lower rate of oxidation of bath
  4. Improved atmospheric corrosion resistance of Zn-coating
  5. Improved shine/brightness of the galvanized surface

Lead:

Presence of lead in the bath has the following major influence in the coating structure & properties:

  1. Increase in spangle size
  2. Dullness in appearance
  3. Lower atmospheric corrosion resistance

It appears from the above that addition of Pb to the bath should be restricted to the least possible extent. However, most of the CGL contain a small quantity of Pb due to

  1. Natural occurrence of Pb in Zn-ore. Complete removal of Pb is not only expensive but also uneconomic in certain conditions.
  2. Market demand of spangles for applications like Roofing, Trunk, A.C. Duct making etc.

Because of low solubility of Pb in Zn, Pb promotes dendritic solidification, leading to distinct spangles on the coated surface Improved Wettability (Wettability is the tendency of one fluid to spread on, or adhere to, a solid surface in the presence of other immiscible fluids)

  1. Higher smoothness & lesser surface defects.

Antimony (pb)

Like Pb, Sb is added to the Zn bath to increase the spangle size. For larger spangles Sb (0.1wt %) is more effective than Pb. Sb addition generally leads to feathery spangles having a combination of bright & dull sections.

Conditions for additives like Pb & Sb for spangle formation :

  1. Limited solubility in solid zinc
  2. Lowering of surface tension of molten Zn
  3. Segregation at the tip of advancing spangle resulting in decrease of tip curvature
  4. Faster growth of spangles.

Nickel – Ni

Ni is generally added to the bath as Zn-Ni alloy ‘
Ni is soluble in the molten Zn at a bath temperature of 450oC.

Ni in Zn bath improves the weldability & paintability of the coatings but makes the product expensive due to high cost of Ni.

Cadmium – Cd

It is a common impurity (~0.1%) in commercial grades of Zn-ingots. Hence some amount of Cd may be present in the bath as impurities.

Although in a Al-Pb free bath Cd can promote spangle formation, in the presence of Al & Pb it may reduce the spangle size and contrast. It has been reported that Cd enhances atmospheric corrosion resistance proportional to its concentration. Small quantity of Cd is reported to have beneficial effect on white rust formation.

ArsenicAs

As is generally regarded as undesirable impurity. Even 0.01% As is considered to be detrimental as it lowers the strength characteristics of the coatings. The bad effects of As may be partially countered by Al-addition.

MagnesiumMg

Small Mg additions appear to increase coating thickness. The effect decreases above 0.6%.

Silicon – Si

Si-reduces the solubility of Fe in Zn bath in the temperature range of 450-460oC through the formation of ferro-silicon, which segregates at the top of the melt. But in the pure Zn bath, addition of same amount of Si accelerates the formation of Fe-Zn IMCs.

Si lowers the Fe-Zn reaction kinetics & improves coating adhesion. Si addition to galvanizing/galfan bath suppresses alloy out-bursting in coatings, thereby improving coating formability. However, controlling the desired Si level in the bath is quite a difficult task. Si helps formation of inhibition layer even in 0.10 wt% Al bath. This IL is quite stable & does not easily break down.

It has been observed that Si-content in the range of 0.01-0.06% alongwith 0.2-0.3% Al lowers the thickness of Zn-Fe alloy layer improving adhesion & mechanical properties of the coating. Si reacts with dissolved Fe & helps retention of Al.

CopperCu

As commercial grades of Zn contain some (<0.05%) Cu, it is as such an unavoidable impurity in the bath. Cu has relatively high solubility (~2%) in molten Zn, hence generally it does not directly contribute to dross generation. About 0.1% Cu does not influence the galvanizing process significantly. Cu at ~0.4% influences coating structure and faster erosion of bath wall leading to higher bottom dross generation.

Note – Higher Cu-content (0.4%-1.25%) neutralizes the effect of 0.15-0.20% Al & causes rapid Zn attack changing the alloy layer & making it more brittle & poorer in appearance.

TinSn

Tin is also a natural impurity to the Zn-bath, sourced from the commercial grades of Zn ingots. However, Sn may be added to the bath in order to

  1. Improve the brightness of coating
  2. Enhance the flowery effect of the spangle
  3. Augment the sales appeal

But in case of CGL, Sn is not generally added as it adversely affects the adhesion properties. It has insignificant influence on corrosion resistance.

Iron – Fe

Sources of Fe in the Zn-bath are :

  1. Pickling salts on the strip surface
  2. Strip
  3. Pot/kettle surface (steel cattle)
  4. Zn-ingots.

Fe has quite low solubility in the molten Zn at the galvanizing temperature, e.g. at 454oC it is 0.03%. The rate of iron accumulation in the bath depends on strip speed.

BismuthBi

Bi facilitates spangle formation & reduces the thickness of the Zn-Fe IMC layers.

Minor Elements

Addition of Minor Elements like Cr, Co, Mn, Ti, V, Zr all tended to neutralize the inhibiting effect of 0.15% Al.

Strip Entry Temperature:

The effect of strip entry temperature is related to both the bath temperature & dipping time. In general with increase in strip entry temperature the Fe-Zn IMC component increases in the alloy layer. The influence is observed to be more pronounced when the bath temperature is higher (490C) or the dipping time is low (<5s).

Dipping Time

Generally the coating thickness increases with the duration of dipping according to a parabolic law for bath temperature up to 470oC. The rate of increase of thickness drops above 470oC bath temperature. The increase in thickness is primarily due to increase in the thickness in alloy layers. A plot of coating thickness against the square root of dipping time gives a straight line which intersect the ordinate at a certain layer thickness. This point represents the thickness of the molten zinc film recovered with the substrate during withdrawal & corresponds to the thickness of the pure zinc layer, provided the continued growth of the alloy layers during cooling is disregarded.

Increase in dipping time increases the coating mass primarily due to increase in the alloy layer, which lowers the ductility & formability of the coating.

Bath Chemistry Control

Most important aspect of bath chemistry control is the control of effective Al. Generally, a mass balance approach for Al & Zn are resorted to. Models are used to predict the effective Al from bath sample and checked with targeted Al. Addition of Al is decided accordingly. Control over both pot Al-content & Al in the coating is critical for the consistent production of automotive quality galvanized & galvannealed strips .

Causes of Pot Al-variation

The major causes of Pot Al-variation are :

  • Control system: Conventional trimming system.
  • Scheduling: Transition from one product to another.
  • Flow patterns:

Transport of Al within the pot is not well understood still now although it depends on

  • Design of inductors & bath components.
  • Pot shape & size.
  • Strip speed & cross section.

Reduction in pot Al-variation results in

  • Uniformity in coating Al.
  • Dross control.

Method of Al-addition

Coating Al-variation is mostly caused by trimming addition (Alzinoy of 10-12% Al). These have a hyper-eutectic composition with a two phase structure consisting of a 5% Al-eutectic & an Al-rich α-phase.

As the bar melts the α-phase floats into the region of the strip, locally enriching Al and resulting in higher coating Al. This enrichment is sporadic, leading to wide variability of Al in the coating.

Controlled addition of Al i.e. replacement of 12% Al Brightener Bars by 0.45-0.75% Al ingots improves the

  1. Uniformity of Al in the bath.
  2. Consistency in coating Al.
  3. Lower dross formation.
  4. Lower Al-consumption

Raw Materials:

In India normally Zinc is available in three grades:

  1. Special High Grade (≥99.99% Zn)
  2. High Grade (≥99.95% Zn)
  3. Prime Western (≥98.5% Zn)

The common impurities found in the commercial grades of steel are Pb, Cd, Fe, Cu, Sn etc. in varying proportions.

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