How To Resolve Zinc Alloy Electroplating Bubbling?

Zinc alloy die-cast components are currently widely used in various decorative applications, such as tie clips, belt buckles, and various metal decorative fasteners. Consequently, high standards are demanded for the surface quality of these castings, along with excellent surface treatment properties. The most common defect in zinc alloy die-castings is surface blistering. When raised blisters appear on the casting surface under the following conditions, they are termed blisters, representing the external manifestation of internal casting defects.

(1) Discovered immediately after die casting.
(2) Revealed after polishing or machining.
(3) Appeared after painting or plating.
(4) Discovered after being left for a period of time.

Causes of Foaming

Causes of Porosity

Primarily due to gas porosity and shrinkage mechanisms. Gas porosity typically appears circular, while shrinkage porosity is often irregular in shape

Causes of Porosity：During metal wave filling and solidification, gas intrusion leads to the formation of cavities on the surface or within the casting. b. Gas emitted from coatings infiltrates the molten metal. c. Excessive gas content in the molten alloy causes gas separation during solidification. When gases trapped in the mold cavity, gases emitted from coatings, and gases released during alloy solidification remain trapped in the casting due to inadequate mold venting, they ultimately form porosity.

Causes of shrinkage cavities: During metal wave solidification, shrinkage cavities form due to volume reduction or failure of the metal wave to compensate for shrinkage in the last solidifying areas. In castings or forgings with uneven thickness, localized overheating causes slower solidification in certain areas, resulting in surface depressions during volume contraction. The presence of gas pores and shrinkage cavities allows water ingress during surface treatment of die-cast parts. When baked after painting or electroplating, gases within the pores expand slightly due to heat, or water inside the pores vaporizes, causing volume expansion. This leads to surface blistering on the castings.

The most common cause of rot

In addition to the high tendency of zinc-based alloys to intergranular corrosion due to the concentration of such harmful impurities as lead, cadmium, and tin in the joints of grain boundaries, the metal matrix itself is destroyed. The electroplating process makes this destruction more critical – affected areas increase in volume in the metal matrix. When lifting the plating layer, it begins to deteriorate on the surface of the castings in the form of blistering. In addition, when operating in a damp environment, intergranular corrosion is more dangerous – it may deform or cause fractures and destruction of the castings.

Caused by cracks

Three major defects of casting are water lines, cold shuts, and hot cracks. Water lines and cold shuts result from the close of a filling sequence. During casting, the initial wave of metal fills the lowest portions of the mold and solidifies against the mold wall before the mold is completely filled. Subsequent waves of molten material entering the cavity fail to fuse with the solidifying layer and overlap the pattern on the joint surface to form a strip defect . When the major portion of these defects occurs in the shallow surface layer of the casting, the defects are referred to as water lines. When they extend into the casting, they are known as cold shuts. Hot cracks are defects caused by uneven casting of wall thickness resulting in solidification stress over the entire surface of the component. Pre-ejection is another factor, often caused by insufficient strength, rough surfaces, and other structural weaknesses caused by uneven forces in ejection . While excessively hot mold temperatures cause a coarser grain structure and formation of interdendritic cracks and other blemishes, the presence of impurities results in a reduction of property. Water marks, cold shuts, or hot cracks may allow the plating solution to ooze into casting defects. This solution vaporizes as the components bake, and the gas pressure causes blisters to occur when enough gas generates to disrupt the plating layer.

Defect Resolution Plan

The controlling of the porosity formation in castings is largely connected with the minimization of gas entrapment. The optimal metal flow should have a free acceleration from the nozzle through the runner and sprue to the mold cavity at high velocity, which results in a smooth and uniformly directed stream. The designed flow pattern is achieved with the tapered runner design as it fastens and optimizes the flow rate gradient from the nozzle to the inner gate. The gas inclusion flaw occurs within the filling system, as the metal flow involving turbulence enters the channel and forms porosity. The previous study on the simulation of parts for die casting uncovered that the sharp angles and increasing cross-sectional area within the runner cause the turbulence to entrain gas. The major factor that helps to eliminate the gas is the possibility of the effective escape from both the runner and mold cavity to the overflow and vent channels.

• For shrinkage cavities: Ensure uniform heat dissipation and simultaneous solidification across all areas during die casting. Avoid shrinkage cavities through proper gate design, gate thickness and placement, mold design, mold temperature control, and cooling.
• For intergranular corrosion: Primarily control harmful impurity content in alloy raw materials, especially lead <0.003%. Pay attention to impurity elements introduced by scrap material.
• For water lines and cold shuts: Increase mold temperature, enhance internal gate velocity, or expand overflow channels in cold-shut zones to minimize cold shut occurrence.
• For hot cracks: Avoid abrupt thickness variations in die-cast parts to reduce stress generation. Adjust relevant die-casting process parameters and lower mold temperature

Causes of Bubbling in Zinc Alloy Electroplating

The surface treatment prior to electroplating is the key reason behind blistering of the zinc alloy. As an amphoteric metal, zinc alloy can interact properly with both the acid and base. The pre-plating process is composed of two primary stages: degreasing and acid pickling. Typically, degreasing is accomplished via immersion in an alkaline degreaser at a temperature of 20 to 40 °C, and acid pickling is a selection of three different acids: hydrochloric acid, sulphuric acid, and nitric acid.

Degreasing is the primary cause of zinc alloy plating blistering: Degreasing involves prolonged immersion and agitation in heated room-temperature degreasers, allowing extended chemical reaction times. Acid pickling, being shorter and using concentrated strong acids with high corrosivity, typically avoids this foaming issue. Consequently, zinc alloys often bypass degreasing before plating, instead undergoing sulfuric acid activation before plating.

Electroplating Process for Zinc Alloys

Due to the dielectric properties of zinc alloy metals, zinc alloys cannot be directly plated with silver, nickel, gold, or similar coatings. Instead, a thicker intermediate copper layer (typically around 5 microns) must be applied first. However, the loose structure of zinc alloys after die casting can also cause blistering. This is considered a die casting defect, though the defect rate for this issue is generally not very high. Therefore, it is crucial to analyze at which specific process stage the bubbling occurs.